using System;

public class Program

{

     static void Main(string[] args)

        {

            //test the string case here

            Console.WriteLine(compressString("aaabccccdd"));

            Console.ReadLine();

        } 

​

        public static string compressString(string input) {

            string compressedString = "";

            int count = 1;

            for (int i = 1; i < input.Length; i++) {

                if (input[i] == input[i-1]) {

                    count++;

                } else {

                    compressedString += input[i - 1];

                    if (count > 1) {

                        compressedString += count;

                    }

                    count = 1;

                }

            }

            compressedString += input[input.Length - 1];

            if (count > 1)

            {

                compressedString += count;

            }

            return compressedString;

        }

}