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<queue>
#include<cmath>
#include<map>
#include<vector>
using namespace std;
typedef long long ll;
ll inf = 100000000;
ll v[205];
ll x[20000005];
ll a[10000005];
ll dp[10005][10005];
int dx[4] = { 1,-1,0,0 };
int dy[4] = { 0,0,1,-1 };
typedef pair<ll, ll> P;


int main() {
	int n;
	cin >> n;
	for (int i = 0; i < n; i++) {
		cin >> a[i];
	}
	sort(a, a + n);
	ll max = -inf, kai=0, count = 0;
	for (int i = 0; i < n; i++) {
		if (a[i] == a[i + 1]) {
			count++;
			if (a[i] >= max) {
				max = a[i];
				kai = count;
			}
		}
		else {
			count = 0;
			
		}
	}
	
	if (kai >= 4) {
		cout << max * 2 << endl;
		return 0;
	}
	else {
		ll big = -inf;
		for (int i = 0; i < n; i++) {
			if (a[i] == a[i + 1]&&a[i]!=max) {
				if (a[i] >= big) {
					big = a[i];
				}
			}
		}
		if (big == -inf) {
			cout << "0" << endl;
			return 0;
		}
		cout << big * max << endl;
		
	}
	
	return 0;
}