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: Runtime Error)

#include <bits/stdc++.h>
using namespace std;
const int MAX_N = 100000;
int sticks[MAX_N + 1];
int N;

int main() {
	ios_base::sync_with_stdio(false);
	memset(sticks, 0, sizeof(sticks));
	cin >> N;
	for (int i = 0; i < N; i++) {
		int st; cin >> st;
		sticks[st]++;
	}
	int area = 0;
	int w = 0;
	for (int i = MAX_N; i >= 1; i--) {
		if (sticks[i] >= 4) {
			area = i * i;
			break;
		}
		else if (sticks[i] >= 2) {
			if (w != 0) {
				area = w * i;
				break;
			}
			w = i;
		}
	}
	cout << area << endl;
	return 0;
}