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 <bits/stdc++.h>
using namespace std;
const long long int mod = 1000000007;


int main() {
	int N, p;
	cin >> N;
	map<int, int>U;
	priority_queue<long long>ans;
	long long po = 0;
	for (size_t i = 0; i < N; i++)
	{
		cin >> p;
		U[p]++;
		if (U.at(p) == 2) { ans.push(p);}
		if (U.at(p) == 4 && ans.top() == p)po = p * p;
	}
	if (ans.size() < 2) {
		cout << 0;
		return 0;
	}
	long long answer = ans.top();
	ans.pop();
	answer *= ans.top();
	cout << max(answer,po);
}