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: Accepted)


#include <iostream>
#include <cstdlib>
#include <string>

int main()
{
    long int L[2] = {2, 1};
     int num;
    int i;
    std::cin >> num;
    
    for(i = 1; i < num; i++){
        switch(i % 2){
            case 1:
                L[0] = L[0] + L[1];
                break;
            case 0:
                L[1] = L[0] + L[1];
                break;
        }
    }
    
    if(i % 2){
        std::cout << L[1];
    }else{
        std::cout << L[0];
    }
    
    return 0;
}