```go
package main

import "fmt"

func main() {
	var n int
	fmt.Scan(&n)
	f := make([]int, n+1)
	for i := 1; i <= n; i++ {
		fmt.Scan(&f[i])
	}

	for i := 1; i <= n; i++ {
		if f[f[f[i]]] == i {
			fmt.Println("YES")
			return
		}
	}
	fmt.Println("NO")
}
```