#include <bits/stdc++.h>   using namespace std;   int euclid(int m, int n) {     int amari;     if ((amari = m % n) == 0) {       return n;     } else {       return euclid(n, amari);     }   }   int main() {     int N;     int A[101];     cin >> N;     for (int i = 1; i <= N; i++) {       cin >> A[i];     }     vector<int> v;     bool used[101] = {};     bool flag = true;     for (int i = 1; i <= N; i++) {       if (!used[i]) {         used[i] = true;         int x = 1;         int now = i;         while (A[now] != i) {           now = A[now];           if (used[now]) {             flag = false;             break;           }           used[now] = true;           x++;         }         v.push_back(x);       }     }     if (!flag) {       cout << -1 << endl;       return 0;     }     int res = 1;     for (int i = 0; i < v.size(); i++) {       int z;       if (v[i] % 2 == 0) {         z = v[i] / 2;       } else {         z = v[i];       }       if (res > z) {         res = res * z / euclid(res, z);       } else {         res = res * z / euclid(z, res);       }     }     cout << res << endl;     return 0;   }   