// Enjoy your stay.

#include <bits/stdc++.h>

#define EPS 1e-9
#define INF 1070000000LL
#define MOD 1000000007LL
#define fir first
#define foreach(it,X) for(auto it=(X).begin();it!=(X).end();it++)
#define ite iterator
#define mp make_pair
#define mt make_tuple
#define rep(i,n) rep2(i,0,n)
#define rep2(i,m,n) for(int i=m;i<(n);i++)
#define pb push_back
#define sec second
#define sz(x) ((int)(x).size())

using namespace std;

typedef istringstream iss;
typedef long long ll;
typedef pair<ll,ll> pi;
typedef stringstream sst;
typedef vector<ll> vi;



int main(){
	cin.tie(0);
	ios_base::sync_with_stdio(0);
	
	int N;
	int a[55];
	cin>>N;
	rep(i,N)cin>>a[i];
	rep(i,N)rep(j,N)if(a[i]==j+1 && a[j]==i+1)return cout<<"Yes"<<endl,0;
	else cout<<"No"<<endl;
}