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 (1.14.1) (judge verdict: Wrong Answer)

// 同じ長さが4本以上あったらその長さで正方形
// 別々の長さの2本以上の組があったら長方形（正方形を除く
// 面積が最大なので当然でかい辺を使う 正方形があるかもしれんので戦わせる
package main

import (
	"fmt"
	"sort"
)

func main() {
	var N int
	fmt.Scan(&N)
	T := make(map[int]int)
	for i := 0; i < N; i++ {
		var V int
		fmt.Scan(&V)
		T[V]++
	}

	sayhooookei := 0
	chyohoookeis := []int{}
	for k, v := range T {
		if v >= 4 {
			sayhooookei = max(sayhooookei, k)
		} else if v >= 2 {
			chyohoookeis = append(chyohoookeis, k)
		}
	}
	sort.Sort(sort.Reverse(sort.IntSlice(chyohoookeis)))
	if len(chyohoookeis) >= 2 {
		chohokei := chyohoookeis[0] * chyohoookeis[1]
		fmt.Println(max(sayhooookei*sayhooookei, chohokei))
	} else if len(chyohoookeis) >= 1 {
		chohooofoooowoooooo := chyohoookeis[0] * sayhooookei
		fmt.Println(chohooofoooowoooooo, sayhooookei)
	} else {
		fmt.Println(sayhooookei)
	}
}
func min(a, b int) int {
	if a < b {
		return a
	}
	return b
}

func max(a, b int) int {
	if a < b {
		return b
	}
	return a
}