CF939A.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.

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

The second line contains $n$ integers $f_{1},f_{2},...,f_{n}$ ( $1<=f_{i}<=n$ , $f_{i}≠i$ ), meaning that the $i$ -th plane likes the $f_{i}$ -th.

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.

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.