B

B - Trained?

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Time limit : 2sec / Memory limit : 256MB

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 ai will be lighten up. It is possible that i=ai. 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≤105
• 1≤ai≤N

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Input

Input is given from Standard Input in the following format:

Na1a2:aN

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.

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Sample Input 1

Copy

3312

Sample Output 1

Copy

2

Press Button 1, then Button 3.

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Sample Input 2

Copy

43412

Sample Output 2

Copy

-1

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

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Sample Input 3

Copy

533424

Sample Output 3

Copy

3

#include <iostream>using namespace std;int main(){    int n;    int a[100000+100];    int b[100000+100];    while(cin>>n){        memset(b,-1,sizeof(b));        for(int i=1;i<=n;i++){            cin>>a[i]; b[i]=0;        }        int flag=0; int k=0;        int i=1;        while(1){            if(b[i]==0){                b[i]=1; k++;                i=a[i];                if(i==2){                  flag=1; break;                }            }            else if(b[i]==1){                flag=0; break;            }        }        if(flag==0)            cout<<-1<<endl;        else            cout<<k<<endl;    }    return 0;}