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: Accepted)

#include <iostream>
#include <algorithm>
using namespace std;
const int N = 100005;
int a[N];
int main()
{
	ios::sync_with_stdio(false);
	int n;
	cin >> n;
	for (int i = 1; i <= n; i++)
		cin >> a[i];
	sort(a + 1, a + n + 1);
	int i = n, w = 0, h = 0;
	for (; i; i--)
		if (a[i] == a[i - 1])
		{
			w = a[i];
			i -= 2;
			break;
		}
	for (; i; i--)
		if (a[i] == a[i - 1])
		{
			h = a[i];
			break;
		}
	cout << 1ll * w * h << endl;
	return 0;
}