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: Accepted)

// Package main provides
//
// File:  c.go
// Author: ymiyamoto
//
// Created on Wed Jan  9 09:22:05 2019
//
package main

import (
	"fmt"
	"sort"
)

func main() {
	var N int
	fmt.Scan(&N)
	arr := make([]int, N)
	for i := 0; i < N; i++ {
		fmt.Scan(&arr[i])
	}
	sort.Ints(arr)

	a, b := 0, 0
	index := 0
	for i := len(arr) - 1; i >= 0; i-- {
		if i > 0 && arr[i] == arr[i-1] {
			a = arr[i]
			index = i - 2
			break
		}
	}

	for i := index; i >= 0; i-- {
		if i > 0 && arr[i] == arr[i-1] {
			b = arr[i]
			break
		}
	}

	fmt.Println(a * b)
}