using System;

using System.IO;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        IEnumerable<int> input = new [] { 1, 3, 5, 7, 9 };

        int result = input

            .Where(i => i%2 == 0)

            .DefaultIfEmpty(-500)

            .SingleOrDefault();

        Console.WriteLine(result);

    }

}