using System;

public class Program

{

    public static void Main()

    {

        var piCode = "PI-1000001";

        var CurrentProformaCode = "PI-1000001_R2";

​

        if(CurrentProformaCode.Contains("_R"))

        {

            var currentId = Convert.ToInt32(CurrentProformaCode.Split('_')[1].ToString().Replace("R", ""));

            var newID = currentId + 1;

            var revicedCode = CurrentProformaCode.Substring(CurrentProformaCode.IndexOf('_'), 1).ToString() + "R" + newID;

            Console.WriteLine(newID);

            Console.WriteLine(CurrentProformaCode);

            Console.WriteLine(revicedCode);

        }

        Console.WriteLine("Hello World");

    }

}