Problem:
Score : 300 points

Problem Statement

We have N sticks with negligible thickness.
The length of the i-th stick is A_i.

Snuke wants to select four different sticks from these sticks and form a rectangle (including a square), using the sticks as its sides.
Find the maximum possible area of the rectangle.

Constraints

4 \leq N \leq 10^5

1 \leq A_i \leq 10^9

A_i is an integer.

Input

Input is given from Standard Input in the following format:

N
A_1 A_2 ... A_N

Output

Print the maximum possible area of the rectangle.
If no rectangle can be formed, print 0.

Sample Input 1

6
3 1 2 4 2 1

Sample Output 1

2

1 \times 2 rectangle can be formed.

Sample Input 2

4
1 2 3 4

Sample Output 2

0

No rectangle can be formed.

Sample Input 3

10
3 3 3 3 4 4 4 5 5 5

Sample Output 3

20

Solution in Go (1.6) (judge verdict: Wrong Answer)

package main

import (
	"fmt"
	"sort"
)

func main() {
	var N, w int
	fmt.Scan(&N)
	m := map[int]int{}
	var a []int
	for i := 0; i < N; i++ {
		fmt.Scan(&w)
		m[w]++
	}
	for k, v := range m {
		if v >= 2 {
			a = append(a, k)
		}
	}
	sort.Sort(sort.Reverse(sort.IntSlice(a)))
	if len(a) > 1 {
		fmt.Println(a[0] * a[1])
	} else {
		fmt.Println(0)
	}
}