#define _CRT_SECURE_NO_WARNINGS #include <bits/stdc++.h> using namespace std; #define mpr make_pair #define pr pair<int,long long> #define st string #define vint vector<int> #define vpr vector<pr> #define fr first #define sc second #define add push_back #define del pop_back #define qdel pop inline int scn() { int hg; cin>>hg; return hg; }; const int MOD = 1000 * 1000 * 1000 + 7; const int N=200006; const long long INF=1000000000000000007; int a[5006]; int main(){ //cout<<INF<<endl; ios_base::sync_with_stdio(false); int n=scn(); if (n==2){ cout<<"NO"<<endl; } for (int i=1;i<=n;i++){ cin>>a[i]; } for (int i=1;i<=n;i++){ if (a[a[a[i]]]==i){ cout<<"YES"<<endl; return 0; } } cout<<"NO"<<endl; return 0; }