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 <algorithm>
#include <string>
#include <cstdio>
#include <cmath>
#include <stack>
#include <cstring>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <numeric>


int main()
{
	long long n;

	std::cin >> n;

	std::map<long long, long long> m;

	for (size_t i = 0; i < n; i++) {
		long long a;
		std::cin >> a;

		m[a]++;
	}

	long long max = 0;
	long long hen = 0;

	for (auto i = m.rbegin(); i != m.crend(); i++) {
		if (i->second < 2) {
			continue;
		}
		if (i->second >= 4) {
			max = i->first * i->first;
			break;
		}
		if (i->second > 1) {
			if (hen == 0) {
				hen = i->first;
			} else {
				max = hen * i->first;
				break;
			}
		}
	}

	std::cout << max << std::endl;

	return 0;
}