Compress string

Given a non-empty string, write a method that returns it in compressed format.
Expected input and output
CompressString("kkkktttrrrrrrrrrr") → "k4t3r10" CompressString("p555ppp7www") → "p153p371w3"
using System;

namespace CSharpExercises
{
    class Program
    {
        public static string CompressString(string str)
        {
            var count = 0;
            var last = str[0];
            var newStr = string.Empty;

            foreach (var s in str)
            {
                if (s == last)
                {
                    count++;
                }
                else
                {
                    newStr += last.ToString() + count;
                    last = s;
                    count = 1;
                }
            }

            newStr += last.ToString() + count;

            return newStr;
        }

        static void Main(string[] args)
        {
            Console.WriteLine(CompressString("aaaabbcccccdaa"));    //a4b2c5d1a2
            Console.WriteLine(CompressString("948kro"));            //914181k1r1o1
            Console.WriteLine(CompressString("$999j*#jjjfYyyy"));   //$193j1*1#1j3f1Y1y3
        }
    }
}