A. Love Triangle

As you could know there are no male planes nor female planes. However, each plane on Earth likes some other plane. There are n planes on Earth, numbered from 1 to n, and the plane with number i likes the plane with number f_i, where 1 ≤ f_i ≤ n and f_i ≠ i.

We call a love triangle a situation in which plane A likes plane B, plane B likes plane C and plane C likes plane A. Find out if there is any love triangle on Earth.

Input

The first line contains a single integer n (2 ≤ n ≤ 5000) — the number of planes.

The second line contains n integers f₁, f₂, ..., f_n (1 ≤ f_i ≤ n, f_i ≠ i), meaning that the i-th plane likes the f_i-th.

Output

Output «YES» if there is a love triangle consisting of planes on Earth. Otherwise, output «NO».

You can output any letter in lower case or in upper case.

Examples Input Copy 5 2 4 5 1 3 Output YES Input Copy 5 5 5 5 5 1 Output NO Note

In first example plane 2 likes plane 4, plane 4 likes plane 1, plane 1 likes plane 2 and that is a love triangle.

In second example there are no love triangles.

题意：求能否找到3个数， 以其中一个数为的下标（从1开始）的数等于

另一个数且这另一个数等于那个数的下标。

#include<bits/stdc++.h> using namespace std; const int N = 5005; int arr[N]; int main(){ int n; cin>>n; for(int i=1;i<=n;i++){ cin>>arr[i]; } int flag=0; for(int i=1;i<=n;i++){ if(arr[arr[arr[i]]]==i){ flag=1; break; } } if(flag) cout<<"YES"<<endl; else cout<<"NO"<<endl; return 0; }