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++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <bits/stdc++.h>
using namespace std;

int main()
{
    int N;
    cin >> N;
    int64_t sum0 = 2;
    int64_t sum1 = 1;
    int64_t sum2 = sum0 + sum1;      // L2 = L1 + L0

    for (int i = 0; i < N-1; i++)
    {
        sum2 = sum1 + sum0;      // L2 = L0 + L1
        sum0 = sum1;            // L0 <- L1
        sum1 = sum2;             // L1 <- L2
        //cout << sum0 << sum1 << sum2 << endl;
    }
    
    cout << sum2 << endl;
}