using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        var vin = "2T3P1RFV2SC494493";

        var v = vin.Substring(11);

        if(!Regex.IsMatch(v, @"[a-zA-Z]{1}"))

            Console.WriteLine("OK!!");

    }

}