#include <bits/stdc++.h>
using namespace std;
long long a[2000009], bb[1000009];
map<long long, long long> mp;
map<long long, long long> mp1;
vector<long long> vc;
int b_search(long long low, long long high, long long m) {
  if (low == high) return low;
  long long mid = (low + high) / 2;
  if ((a[mid] < m))
    b_search(mid + 1, high, m);
  else
    b_search(low, mid, m);
}
int main() {
  cin.sync_with_stdio(false);
  long long i, j, k, l, c, v, b, bv[1000], aa[1009], bb1[1000], n, m, r, t, e,
      f;
  long long x1, x2, y1, y2, x3, x4, x5, x6;
  string s, s1, s2, s3, ss[10000];
  while (cin >> n) {
    for (i = 1; i <= n; i++) {
      cin >> a[i];
    }
    for (i = 1; i <= n; i++) {
      if (a[a[a[a[i]]]] == a[i]) {
        cout << "YES" << endl;
        return 0;
      }
    }
    cout << "NO" << endl;
  }
}
