Problem:
Score : 200 points

Problem Statement

It is November 18 now in Japan. By the way, 11 and 18 are adjacent Lucas numbers.

You are given an integer N. Find the N-th Lucas number.

Here, the i-th Lucas number L_i is defined as follows:

L_0=2

L_1=1

L_i=L_{i-1}+L_{i-2} (i≥2)

Constraints

1≤N≤86

It is guaranteed that the answer is less than 10^{18}.

N is an integer.

Input

Input is given from Standard Input in the following format:

N

Output

Print the N-th Lucas number.

Sample Input 1

5

Sample Output 1

11

L_0=2

L_1=1

L_2=L_0+L_1=3

L_3=L_1+L_2=4

L_4=L_2+L_3=7

L_5=L_3+L_4=11

Thus, the 5-th Lucas number is 11.

Sample Input 2

86

Sample Output 2

939587134549734843

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
	static void Main()
	{
		var n = int.Parse(Console.ReadLine());

		if (n == 1) {
			Console.WriteLine(1);
		} else {
			long n1 = 2;
			long n2 = 1;
			for (var i = 1; i < n; ++i) {
				long temp = n2;
				n2 = n1 + n2;
				n1 = temp;
			}

			Console.WriteLine(n2);
		}

		//デバッグ実行での確認用。そのまま提出しても問題なし
		Console.ReadKey();
	}
}