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