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 main(void){
	int n;
	int tmp;
	int cnt = 1;
	vector<int>a,b;
	cin >> n;
	for(int i = 0; i < n; i++){
		cin >> tmp;
		a.push_back(tmp);
	}
	a.push_back(0);
	sort(a.begin(),a.end(),greater<int>());
	tmp = a[0];
	for(int i = 1; i <= n; i++){
		if(tmp == a[i]) cnt++;
		else {
			//cout << "a[i] = " << a[i] << endl;
			if(b.size() == 2) break;
			if(cnt == 4) {b.push_back(a[i-1]);b.push_back(a[i-1]);break;}
			else if (cnt >= 2) b.push_back(a[i-1]);
			cnt = 1;
			tmp = a[i];
		}
		//cout << "i = " <<i<<" cnt = "<< cnt<< " a["<<i<<"] = "<< a[i]<<endl;
	}
		if(b.size() == 2) cout << b[0] * b[1] << endl;
		else cout << "0" << endl;
		//for(int i = 0; i < b.size(); i++) cout << b[i] << endl;
	return 0;
}