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: Accepted)

#include<bits/stdc++.h>
using lint = long long;
using std::vector;
std::map<lint, int> cnt;
int main() {
	int n;
	scanf("%d", &n);
	for (int i = 0; i < n; i++) {
		lint a;
		scanf("%lld", &a);
		cnt[a]++;
	}
	lint ans = 0;
	vector<lint> p;
	for (auto q : cnt) {
		if (q.second >= 4) {
			ans = std::max(ans,q.first*q.first);

		}
		if (q.second >= 2) {
			p.push_back(q.first);
		}
	}
	std::sort(p.begin(), p.end(), std::greater<lint>());
	if (p.size() >= 2) {
		ans = std::max(ans, p[0] * p[1]);
	}
	printf("%lld", ans);
}