Shopping Center With Data Table (time limits)

using System;
using System.Data;
using System.Text;

namespace ShoppingCenter
{
    public class ShoppingCenter
    {
        private const string ProductAdded = "Product added";

        private const string XProductsDeleted = " products deleted";

        private const string NOProductsFound = "No products found";

        private const string IncorrectCommand = "Incorrect command";

        public static DataTable center = new DataTable();

        public static void Main()
        {
            // first row contains column names:
            center.Columns.Add("Product");
            center.Columns.Add("Producer");
            center.Columns.Add("Price");
            center.Columns["Price"].DataType = typeof(decimal);
            center.CaseSensitive = true; // Case-sensitive data tables require exact match of searched data. e.g. Forest & forest are 2 different names when this property is set to true.

            StringBuilder answer = new StringBuilder();

            int commands = int.Parse(Console.ReadLine());

            for (int i = 1; i <= commands; i++)
            {
                string command = Console.ReadLine();

                string commandResult = ProcessCommand(command);

                answer.AppendLine(commandResult);
            }

            Console.Write(answer);
        }

        public static string ProcessCommand(string command)
        {
            int indexOfFirstSpace = command.IndexOf(' ');

            string method = command.Substring(0, indexOfFirstSpace);

            string parameterValues = command.Substring(indexOfFirstSpace + 1);

            string[] parameters = parameterValues.Split(new char[] { ';' }, StringSplitOptions.RemoveEmptyEntries);

            string commandResult;

            switch (method)
            {
                case "AddProduct":
                    {
                        commandResult = AddProduct(parameters[0], parameters[1], parameters[2]);

                        break;
                    }

                case "DeleteProducts":
                    {
                        if (parameters.Length == 1)
                        {
                            commandResult = DeleteProducts(parameters[0]);
                        }
                        else
                        {
                            commandResult = DeleteProducts(parameters[0], parameters[1]);
                        }

                        break;
                    }

                case "FindProductsByName":
                    {
                        commandResult = FindProductsByName(parameters[0]);

                        break;
                    }

                case "FindProductsByPriceRange":
                    {
                        commandResult = FindProductsByPriceRange(parameters[0], parameters[1]);

                        break;
                    }

                case "FindProductsByProducer":
                    {
                        commandResult = FindProductsByProducer(parameters[0]);

                        break;
                    }

                default:
                    {
                        commandResult = IncorrectCommand;

                        break;
                    }
            }

            return commandResult;
        }

        private static string FindProductsByProducer(string producer)
        {
            string filter = string.Format("Producer='" + producer + "'");
            var rows = center.Select(filter, "Product ASC, Producer ASC, Price ASC");

            int counter = 0;
            StringBuilder results = new StringBuilder();
            foreach (var row in rows)
            {
                results.Append("{");
                results.AppendFormat("{0};{1};{2:f2}", row[0], row[1], row[2]);
                results.Append("}");
                results.AppendLine();
                counter++;
            }

            if (counter == 0)
            {
                return NOProductsFound;
            }
            else
            {
                results.Length--;
                results.Length--;
                return results.ToString();
            }
        }

        private static string FindProductsByPriceRange(string priceFrom, string priceTo)
        {
            string filter1 = string.Format("Price>=" + priceFrom);
            string filter2 = string.Format("Price<=" + priceTo);
            string filter = filter1 + " AND " + filter2;
            var rows = center.Select(filter, "Product ASC, Producer ASC, Price ASC");

            int counter = 0;
            StringBuilder results = new StringBuilder();
            foreach (var row in rows)
            {
                results.Append("{");
                results.AppendFormat("{0};{1};{2:f2}", row[0], row[1], row[2]);
                results.Append("}");
                results.AppendLine();
                counter++;
            }

            if (counter == 0)
            {
                return NOProductsFound;
            }
            else
            {
                results.Length--;
                results.Length--;
                return results.ToString();
            }
        }

        private static string FindProductsByName(string product)
        {
            string filter = string.Format("Product='" + product + "'");
            var rows = center.Select(filter, "Product ASC, Producer ASC, Price ASC");

            int counter = 0;
            StringBuilder results = new StringBuilder();
            foreach (var row in rows)
            {
                results.Append("{");
                results.AppendFormat("{0};{1};{2:f2}", row[0], row[1], row[2]);
                results.Append("}");
                results.AppendLine();
                counter++;
            }

            if (counter == 0)
            {
                return NOProductsFound;
            }
            else
            {
                results.Length--;
                results.Length--;
                return results.ToString();
            }
        }

        private static string DeleteProducts(string producer)
        {
            string filter = string.Format("Producer='" + producer + "'");
            var rows = center.Select(filter);
            int counter = 0;
            foreach (var row in rows)
            {
                row.Delete();
                counter++;
            }

            if (counter == 0)
            {
                return NOProductsFound;
            }
            else
            {
                return counter + XProductsDeleted;
            }
        }

        private static string DeleteProducts(string product, string producer)
        {
            string filter1 = string.Format("Producer='" + producer + "'");
            string filter2 = string.Format("Product='" + product + "'");
            string filter = filter1 + " AND " + filter2;
            var rows = center.Select(filter);
            int counter = 0;
            foreach (var row in rows)
            {
                row.Delete();
                counter++;
            }

            if (counter == 0)
            {
                return NOProductsFound;
            }
            else
            {
                return counter + XProductsDeleted;
            }
        }

        private static string AddProduct(string product, string price, string producer)
        {
            DataRow row = center.NewRow();
            row[0] = product;
            row[1] = producer;
            row[2] = decimal.Parse(price);
            center.Rows.Add(row);

            return ProductAdded;
        }
    }
}