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;

int N;
vector<int> E;

int main() {
	cin >> N;
	for (int i = 0; i < N; i++) {
		int a; cin >> a;
		E.push_back(a);
	}
	sort(E.begin(), E.end());

	int l1= 0;
	for (int i = E.size() - 2; i >= 0; i--) {
		if (E[i] == E[i + 1] && l1 == 0) {
			l1 = E[i];
			break;
		}
	}

	int l2 = 0;
	for (int i = E.size() - 2; i >= 0; i--) {
		if (E[i] == E[i + 1] && l2 == 0 && E[i] != l1) {
			l2 = E[i];
			break;
		}
	}
	cout << l1*l2 << endl;
}