#include <bits/stdc++.h>
using namespace std;
int main(){
  int N;
  cin >> N;
  vector<int> A(N);
  for (int i = 0; i < N; i++){
    cin >> A[i];
    A[i]--;
  }
  vector<bool> used(N, false);
  int cnt = 0;
  for (int i = 0; i < N; i++){
    if (!used[i]){
      used[i] = true;
      int j = i;
      while (true){
        j = A[j];
        if (used[j]){
          break;
        }
        used[j] = true;
      }
      cnt++;
    }
  }
  if (cnt % 2 == N % 2){
    cout << 1 << endl;
  } else {
    cout << -1 << endl;
  }
}