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 C++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <iostream>
#include <cstdio>
#include <stdio.h>
#include <algorithm>
#include <functional>

using namespace std;

int x[1000 * 1000 * 10] = { 0 };

int main() {
	int n, a = 0, s[2] = { 0,0 }, i, res = 0;
	cin >> n;
	for (i = 0; i<n; i++) {
		cin >> x[i];
	}

	sort(x, x + n,greater<int>());
	
	a = 0;
	i = 0;

	while (i < n) {
		s[a] = x[i]; i++;
		if (i >= n) break;
		if (s[a] == x[i]) {
			a++;
			i++;
			if (i >= n) break;
		}
		if (a >= 2) {
			res = s[0] * s[1];
			break;
		}
	}

	cout << res << endl;
	return 0;
}