#include <bits/stdc++.h>   using namespace std;   int main() {     int t, n, m, x;     unordered_map<int, int> mp;     cin >> t;     int a[t];     for (int i = 1; i <= t; i++) {       cin >> a[i];       mp[i] = a[i];     }     for (int i = 1; i <= t; i++) {       n = mp[i];       m = mp[n];       x = mp[m];       if (i == x) {         cout <<  YES ;         return 0;       }     }     cout <<  NO  << endl;   }   