C# - Grade Calculation

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConditionalStatementApp
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Enter 1st Number: ");
            int number = Convert.ToInt32(Console.ReadLine());
            Console.WriteLine("Your Grade Is:" + CalculateGrade(number));
        }

         static string CalculateGrade(int number)
        {
            if (number >= 90 && number <= 100)
            {
                Console.WriteLine("A+");
            }
            else if (number >= 80 && number <= 89)
            {
                Console.WriteLine("A-");
            }
            else if (number >= 70 && number <= 79)
            {
                Console.WriteLine("B+");
            }
            else if (number >= 60 && number <= 69)
            {
                Console.WriteLine("C+");
            }
            else if (number >= 50 && number <= 59)
            {
                Console.WriteLine("D+");
            }
            else if (number >= 0 && number <= 49)
            {
                Console.WriteLine("F");
            }

            else
            {
                Console.WriteLine("Invalid");
            }

            Console.ReadKey();

        }
    }
}