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 (1.15.1) (judge verdict: Accepted)

use std::io;

macro_rules! read {
    ($t: ty) => {
        {
            let mut input = String::new();
            io::stdin().read_line(&mut input).expect("could not read input");
            input.trim().parse::<$t>().unwrap()
        };
    };
    ($($t: ty),*) => {
        {
            let mut input = String::new();
            io::stdin().read_line(&mut input).expect("could not read input");
            let mut iter = input.trim().split_whitespace();
            ($(iter.next().unwrap().parse::<$t>().unwrap()), *)
        };
    }
}
macro_rules! read_vec {
    ($t: ty) => {
        {
            let mut input = String::new();
            io::stdin().read_line(&mut input).expect("could not read input");
            let iter = input.trim().split_whitespace();
            iter.map(|x| x.parse::<$t>().unwrap()).collect::<Vec<$t>>()
        };
    }
}
fn main() {

    let n = read!(u32);

    if n == 1 {
        println!("1");
        return;
    }

    let mut prevprev: u64 = 2;
    let mut prev: u64 = 1;
    let mut ln: u64 = 0;

    for i in 2..n+1 {
        ln = prevprev + prev;
        prevprev = prev;
        prev = ln;
    }
    println!("{}", ln);
}