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 C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include<iostream>
#include<vector>
#include<string>

using namespace std;

int cnt(vector<int>A){
  
  int memo[100010] = {0}; int i = 1; int count = 0;
  bool flag = false;
  
  while(memo[i] <= 1){
    memo[i]++; count++;
       if(A[i] == 2){ flag = true; break;}
       else{ i = A[i]; continue;}}
      
 
 if(flag){return count;}
 else{return -1;}}
  

int main(){
  int N; cin >> N; vector<int> A(N + 1); 
  for(int i = 1; i <= N ; i++){cin >> A[i];}
  //switchの番号は1-indexで扱う
  cout << cnt(A) << endl;
  
  return 0;}