IdentifierGenerator.cs

public static class IdentifierGenerator
{
    private const string Alphabet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
    private static readonly int _base = Alphabet.Length;

    public static string Encode(int index)
    {
        if (index == 0)
            return Alphabet[0].ToString();

        var encoded = string.Empty;
        while (index > 0)
        {
            encoded += Alphabet[index % _base];
            index = index / _base;
        }

        return string.Join(string.Empty, encoded.Reverse());
    }
}

Program.cs

public class Program
{
    private static void Main()
    {
        while (true)
        {
            string input = Console.ReadLine();
            int id = int.Parse(input);
            Console.WriteLine("http://easysha.re/{0}", IdentifierGenerator.Encode(id));
        }
    }
}