using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        //foreach (var num in fl(20))

        //  Console.Write(num + " ");

        //Console.WriteLine();

        //Console.WriteLine(fn(10));

    }

    public static IEnumerable<int> fl(int len)

    {

        int a = 1, b = 1, c = 0;

        yield return a;

        yield return b;

​

        for (int i = 2; i < len; i++)

        {

            c = a + b;

            yield return c;

            a = b;

            b = c;

        }

    }

    public static int fn(int n)

    {

        if ((n == 0) || (n == 1))

            return 1;

        else

            return fn(n - 1) + fn(n - 2);

    }

}