S

#include <iostream>

using namespace std;

int main() {
    int n;
    cin >> n;
    int f[n];
    bool triangle = false;

    for (int i = 0; i < n; i++) {
        cin >> f[i];
    }

    for (int i = 0; i < n - 2; i++) {
        int checker[4] = { 0,0,0,0 };
        checker[0] = i + 1;
        checker[1] = f[checker[0] - 1];
        checker[2] = f[checker[1] - 1];
        checker[3] = f[checker[2] - 1];

        if (checker[0] == checker[3]) {
            triangle = true;
        }
    }

    if (triangle) {
        cout << "YES\n";
    }
    else {
        cout << "NO\n";
    }
}