#include <bits/stdc++.h>
using namespace std;
const long long inf = (1LL << 60) - 1;
const long long mxn = 1e5;
int dx[] = {-1, 1, 0, 0};
int dy[] = {0, 0, -1, 1};
long long ans, a[100005];
long long mod(long long x) {
  long long res = x % 1000000007;
  while (res < 0) res += 1000000007;
  return res;
}
long long bigmod(long long x, long long n) {
  long long res = 1;
  while (n > 0) {
    if (n & 1) res = (res * x) % 1000000007;
    x = (x * x) % 1000000007;
    n >>= 1;
  }
  return res;
}
void solve() {
  long long n;
  cin >> n;
  vector<long long> a(n + 1), b(n + 1);
  set<long long> s;
  for (long long i = 1; i <= n; i++) cin >> a[i];
  for (long long i = 1; i <= n; i++) {
    long long x = a[i];
    if (x == a[a[a[x]]]) {
      cout << "YES"
           << "\n";
      return;
    }
  }
  cout << "NO"
       << "\n";
}
int main() {
  ios::sync_with_stdio(0);
  cout.tie(0);
  ;
  solve();
  return 0;
}
