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 Rust (1.42.0) (judge verdict: Accepted)

use proconio::{input, fastout};
use std::process;

#[fastout]
fn main() {
    input!{
        n: usize,
    }
    let mut ret: u64 = 0;
    let mut prev: u64 = 2;
    let mut curr: u64 = 1;
    if n == 1 {
	println!("1");
    } else {
	for i in 2..(n+1) {
	    ret = prev + curr;
	    prev = curr;
	    curr = ret;
	}
	println!("{}", ret);
    }
}