using System;
using System.Text;
using System.Collections.Generic;
using System.Text.RegularExpressions;

public class Test
{
	private static string[] productCodes = new[] { 
		"MD0133776311I",
		"M10133776311I",
		"M20133776311I"
	};

	private static Dictionary<char, char> replacementMap = new Dictionary<char, char> {
		{ '1', 'I' },
		{ '2', 'S' },
	};

	private static Regex startsWithTwoLettersRegex = new Regex("^([A-Z]{2})");

	public static void Main()
	{
		foreach (var code in productCodes)
		{
			Console.Write("{0} -> ", code);
			if (!startsWithTwoLettersRegex.Match(code).Success)
			{
				Console.WriteLine(FixProductCode(code));
			}
			else
			{
				Console.WriteLine("OK!");	
			}
		}
	}
	
	static string FixProductCode(string code)
	{
		StringBuilder sb = new StringBuilder(code);
		sb[1] = replacementMap[code[1]];
		return sb.ToString();
	}
}