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 PyPy3 (7.3.0) (judge verdict: Accepted)

def solve():
    N = int(input())
    L0 = 2
    L1 = 1
    if N == 1:
        print(L1)
        exit()
    Li2 = L0
    Li1 = L1
    Li = Li1 + Li2
    for _ in range(N - 2):
        Li2 = Li1
        Li1 = Li
        Li = Li1 + Li2
    print(Li)

if __name__ == "__main__":
    solve()