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::{stdin, Read, StdinLock};
use std::str::FromStr;
use std::cmp;

struct Scanner<'a> {
    cin : StdinLock<'a>,
}

impl<'a> Scanner<'a> {
    fn new(cin : StdinLock<'a>) -> Scanner<'a> {
        Scanner { cin: cin }
    }

    fn read1<T: FromStr>(&mut self) -> Option<T> {
        let token = self.cin.by_ref().bytes().map(|c| c.unwrap() as char)
            .skip_while(|c| c.is_whitespace())
            .take_while(|c| !c.is_whitespace())
            .collect::<String>();
        token.parse::<T>().ok()
    }

    fn read<T: FromStr>(&mut self) -> T {
        self.read1().unwrap()
    }
}

fn main(){
	let cin = stdin();
	let cin = cin.lock();
	let mut sc = Scanner::new(cin);
	let n : usize = sc.read();
        let mut arr : Vec<usize>  = Vec::new();
        arr.push(0);
        for _ in 0..n {
            arr.push(sc.read());
        }
        let mut i = 1;
        let mut cnt = 0;
        loop {
            let next = arr[i];
            arr[i] = 0;
            match next {
                0 => {
                    cnt = 0;
                    break;
                },
                2 => {
                    cnt += 1;
                    break;
                },
                _ => {
                    i = next;
                    cnt += 1;
                }
            }
        }
        println!("{}", if cnt == 0 {
            -1
        } else {
            cnt
        });
}