[TNI] Random Generated String

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

namespace ConsoleApp6
{
    class Program
    {
        public static String ricardo(String elements, int n)
        {
            Random random = new Random();
            int length = elements.Length;

            //make string into char array
            char[] elems = elements.ToCharArray();

            //final char array
            char[] newS = new char[n];

            //the actual task
            for (int i = 0; i < n; i++)
            {
                newS[i] = elems[random.Next(0, length)];
            }
            //have to StringBuilder, u cant .ToString() a char array for some reason
            StringBuilder sb1 = new StringBuilder();
            for (int i = 0; i < newS.Length; i++)
            {
                sb1.Append(newS[i]);
            }

            String final = sb1.ToString();

            return final;
        }
        static void Main(string[] args)
        {
            //make sure to enter a number first
            int M = 0;
            Console.WriteLine("Enter M");
            M = Convert.ToInt32(Console.ReadLine());
            StringBuilder sb = new StringBuilder();
            Console.WriteLine("Enter " + M + " chars");

            //enter as many chars as the previous entered number, form a string
            while (M != 0)
            {
                String s = Console.ReadLine();
                sb.Append(s);
                M--;
            }
            String tmp = "";
            tmp = sb.ToString();

            //after u enter chars enter how big the random string u want it to be
            int n = 0;
            Console.WriteLine("Enter n");
            n = Convert.ToInt32(Console.ReadLine());
            String finale = "failed";
            finale = ricardo(tmp, n);
            Console.WriteLine(finale);
            Console.ReadKey();
        }
        }
    }