// Enjoy your stay. #include #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 pi; typedef stringstream sst; typedef vector 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"<