without a loop in C#, so O(1) time and complexity https://math.stackexchange.com/questions/90821/how-to-find-the-closed-form-to-the-fibonacci-numbers

using System;
using System.Collections.Generic;
using System.IO;

class Solution {

    public static int Fibonacci(int n) {
        double sqrt_5 = Math.Sqrt(5);
        double inv_sqrt_5 = 1 / sqrt_5;
        double part_one = inv_sqrt_5 * Math.Pow((1 + sqrt_5) / 2, n);
        double part_two = inv_sqrt_5 * Math.Pow((1 - sqrt_5) / 2, n);
        return Convert.ToInt32(part_one - part_two);
    }

        static void Main(String[] args) {
                int n = Convert.ToInt32(Console.ReadLine());
                Console.WriteLine(Fibonacci(n));
        }
}