using System;

using System.Linq;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(f(1)); //returns 2.

        Console.WriteLine(f(665)); //returns 667.

        Console.WriteLine(f(665999999)); //returns 667000000 without having looped a million times.

        //(Following examples added since the question was first posed.)

        Console.WriteLine(f(666666666)); //also returns 667000000.

        Console.WriteLine(f(66600)); // returns 66700.

    }

    public static int f(int b)

    {

        b++;

        var z = new Regex("666[\\d]*");

        var q = z.Replace(b+"", "667", 1);

        return Int32.Parse(q.PadRight((b+"").Length, '0')); 

    }

}