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 <vector>
#include <algorithm>

using namespace std;

int main()
{
	int n;
	cin >> n;
	
	vector<int> a(n);
	for(int i = 0; i < n; i++)
	{
		cin >> a[i];
	}
	
	sort(a.begin(), a.end(), [](const int& a, const int& b) {return a > b;});
	
	int i = 0;
	int max1 = 0;
	int max2 = 0;
	while(i < n)
	{
		int cnt = 0;
		while(a[i] == a[i + cnt] && (i + cnt) < n)
		{
			cnt++;
			if(cnt == 2)
			{
				max1 = max2;
				max2 = a[i];
				break;
			}
		}
		if(max1 != 0)
		{
			break;
		}
		
		i += cnt;
	}
	
	if(max1 == 0)
	{
		cout << 0 << endl;
	}
	else
	{
		cout << max1 * max2 << endl;
	}
	return 0;
}