Problem:
Score : 200 points

Problem Statement

Takahashi wants to gain muscle, and decides to work out at AtCoder Gym.

The exercise machine at the gym has N buttons, and exactly one of the buttons is lighten up.
These buttons are numbered 1 through N.
When Button i is lighten up and you press it, the light is turned off, and then Button a_i will be lighten up. It is possible that i=a_i.
When Button i is not lighten up, nothing will happen by pressing it.

Initially, Button 1 is lighten up. Takahashi wants to quit pressing buttons when Button 2 is lighten up.

Determine whether this is possible. If the answer is positive, find the minimum number of times he needs to press buttons.

Constraints

2 ≤ N ≤ 10^5

1 ≤ a_i ≤ N

Input

Input is given from Standard Input in the following format:

N
a_1
a_2
:
a_N

Output

Print -1 if it is impossible to lighten up Button 2.
Otherwise, print the minimum number of times we need to press buttons in order to lighten up Button 2.

Sample Input 1

3
3
1
2

Sample Output 1

2

Press Button 1, then Button 3.

Sample Input 2

4
3
4
1
2

Sample Output 2

-1

Pressing Button 1 lightens up Button 3, and vice versa, so Button 2 will never be lighten up.

Sample Input 3

5
3
3
4
2
4

Sample Output 3

3

Solution in Rust (1.15.1) (judge verdict: Accepted)

use std::io;
use std::str::FromStr;

fn main() {
    let stdin = io::stdin();
    let mut buf = String::new();
    stdin.read_line(&mut buf).ok();

    let mut it = buf.split_whitespace().map(|n| usize::from_str(n).unwrap());
    let n = it.next().unwrap();
    let mut v = Vec::with_capacity(n);
    for _ in 0..n {
        let mut buf = String::new();
        stdin.read_line(&mut buf).ok();

        let mut it = buf.split_whitespace().map(|n| usize::from_str(n).unwrap());
        let mut a = it.next().unwrap();
        v.push(a - 1);
    }

    let mut ans = 0;
    let mut next: usize = v[0];
    for _ in 0..n {
        //        println!("next => {}", next);
        if next == 1 {
            println!("{}", ans + 1);
            return;
        }
        next = v[next];
        ans += 1;
    }
    println!("{}", -1);
    return;
}