#include <bits/stdc++.h>
using namespace std;
long long binpow(long long a, long long n) {
  long long res = 1;
  while (n) {
    if (n % 2) res *= a;
    a *= a;
    n /= 2;
  }
  return res;
}
template <typename T>
T sqr(T x) {
  return x * x;
}
template <typename T>
void vout(T s) {
  cout << s << "\n";
  exit(0);
}
long long log(long long a, long long b) {
  long long res = 0;
  while (a) {
    a /= b;
    res++;
  }
  return res;
}
const long long MAXN = 1123456;
vector<long long> v[MAXN];
set<pair<long long, long long> > st;
stringstream ss;
set<long long> s;
string s2, s1, s3;
char cc;
long long c, sc, n, m, i, j, a[MAXN], b[MAXN], a1, b1, c1, ans, x, k;
bool f[MAXN];
void dfs(long long x, long long stn) {
  if (x == i && stn == 3) vout("YES");
  if (f[x]) return;
  f[x] = 1;
  long long i, nn = v[x].size();
  for (i = 0; i < nn; i++) dfs(v[x][i], stn + 1);
  return;
}
int main() {
  ios_base ::sync_with_stdio(0);
  cin.tie(0);
  cin >> n;
  for (i = 1; i <= n; i++) {
    cin >> x;
    v[i].push_back(x);
  }
  for (i = 1; i <= n; i++) {
    for (j = 1; j <= n; j++) f[j] = 0;
    dfs(i, 0);
  }
  cout << "NO"
       << "\n";
  return 0;
}
