Fibonacci

        public static class Fibonacci
        {
            public static IEnumerable<int> GetNumbers()
            {
                yield return 0;

                int current = 1;
                int previous = 0;

                while (true)
                {
                    yield return current;
                    current += previous;
                    previous = current - previous;
                }
            }
        }

        class Program
        {
            static void Main(string[] args)
            {
                int count = Fibonacci.GetNumbers().Where(x => x % 2 == 1).TakeWhile(x => x < 4000000).Count();
                Console.WriteLine(count);
            }
        }