import java.util.*;

 public class Main {

     public static final int maxn = 5000;

     public static int [] v = new int [maxn+10];

     public static int [] f = new int [maxn+10];

     public static int DFS(int x, int t) {

         if (v[x]>0) return t - v[x];

         v[x] = t;

         int k = DFS(f[x], t+1);

         v[x] = 0;

         return k;

     }

     public static void main(String[] args) {

         Scanner input = new Scanner(System.in);

         int n = input.nextInt();

         for (int i = 1; i<=n; i++) f[i] = input.nextInt();

         boolean flag = true;

         for (int i = 1; i<=n; i++) {

             if (3 == DFS(i, 1)) {

                 System.out.println("YES");

                 flag = false;

                 break;

             }

         }

         if(flag) System.out.println("NO");

         input.close();

     }

 }