#include <bits/stdc++.h>   using namespace std;   int main() {     int N;     cin >> N;     vector<int> adjL(N, -1);     for (int n = 0; n < N; n++) {       cin >> adjL[n];       adjL[n]--;     }     bool c = false;     for (int i = 0; i < N; i++) {       int node = i;       for (int x = 0; x < 3; x++) {         if (adjL[node] != -1) {           node = adjL[node];         }       }       if (node == i) {         cout <<  YES ;         c = true;         break;       }     }     if (!c) cout <<  NO ;     return 0;   }   