using System;

public class Program

{

    public static void Main()

    {string  value= "ttt_eeee_yyysachintttttttttfff_add";

​

        // Part 1: get index of separator.

        var i= value.IndexOf("_add");

​

         var S2 = value.Remove(i,value.Length-i);

        // Part 2: if separator exists, get substring.

        if (i >= 0)

        {

            Console.WriteLine("RESULT: {0}", S2);

        }

        Console.WriteLine("Hello World");

    }

}