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::*;
use std::str::FromStr;

fn main() {
    let mut strs = String::new();
    let stdin = stdin();
    let mut stdinlock = stdin.lock();
    stdinlock.read_line(&mut strs).unwrap();
    let n:i64 = i64::from_str(strs.trim()).unwrap();
    let mut backloga = 1u64;
    let mut backlogb = 2u64;
    for _ in 1..n {
        let test:u64 = backloga + backlogb;
        backlogb = backloga;
        backloga = test;
    }
    println!("{}", backloga);
}