using System;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		var str = "74Mercedes1980";
		var n = 3;
		var ch = 'H';
		var regex = new Regex("(\\S{" + (n - 1) + "})\\S");
		str = regex.Replace(str, "$1" + ch);
		Console.WriteLine(str);
	}
}