#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, f = 0;
  cin >> n;
  int a[n + 1];
  for (int i = 1; i <= n; i++) cin >> a[i];
  for (int i = 1; i < n; i++) {
    if (a[a[a[i]]] == i) {
      f = 1;
      break;
    }
  }
  if (f == 1)
    cout << "YES";
  else
    cout << "NO";
}
