using System;

using System.Text.RegularExpressions;                   

public class Program

{

    public static void Main()

    {

      var input = "MyApp v2020.8.3.1.exe";

      var result = Regex.Replace(input, @"(\d+)\.(\d+)\.(\d+)\.(\d+)", "$1.$2.$3-r$4");

      Console.WriteLine(result);

    }

}