fib.cs

using System;
using System.Numerics;

class Program
{
    private static BigInteger fib(int n)
    {
        Func<BigInteger, BigInteger, int, BigInteger> fn = null;
        fn = (x, y, i) => (i > 0 ? fn(y, x + y, i - 1) : y);
        return fn(new BigInteger(1), new BigInteger(0), n);
    }

    static void Main(string[] args)
    {
        string input;
        while ((input = Console.ReadLine()) != "")
        {
            int n;
            if (!int.TryParse(input, out n)) break;
            Console.WriteLine(fib(n));
        }
    }
}