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)

fn read<T: std::str::FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().parse().ok().unwrap()
}

fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    read::<String>().split_whitespace()
                    .map(|e| e.parse().ok().unwrap())
                    .collect()
}

fn main() {
    let n: u32 = read();

    let l0 = 2;
    let l1 = 1;
    let mut l = l0+l1;

    let mut ans: u64 = 0;

    if n == 0 {
        ans = 2;
    } else if n == 1 {
        ans = 1;
    } else {
        let mut prev = l1;
        let mut prev2 = l0;
        for _ in 2..(n+1) {
            l = prev + prev2;
            prev2 = prev;
            prev = l;
        }
        ans = l;
    }

    println!("{}", ans);

}