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 Scala (2.11.7) (judge verdict: Accepted)

import scala.io.StdIn
import scala.collection.mutable.Buffer
import scala.annotation.tailrec

object Main extends App {
  val n = StdIn.readInt
  val a = Iterator.continually(StdIn.readInt)
    .take(n)
    .toArray
    .toBuffer
  0 +=: a

  val end = 2
  val visit = Array.ofDim[Boolean](n+1)
  @tailrec
  def rec(cnt:Int, from:Int):Int = {
      val to = a(from)
      if(to == end) cnt
      else if(visit(from)) return -1
      else {
        visit(from) = true
        rec(cnt+1, to)
      }
  }

  val ans = rec(1, 1)

  println(ans)
}