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(){
	int n,a[100010]={};
	cin >> n;
	for(int i=0;i<n;i++){
		cin >> a[i];
	}
	sort(a,a+100010);
	reverse(a,a+100010);
	for(int i=0;i<n;i++){
		cout << i << ":" << a[i] << endl;
	}
	int ans=1;
	int cnt=0;
	for(int i=0;i<n-1;i++){
		if(a[i]==a[i+1]){
			if(cnt<2){
				ans*=a[i];
				a[i]=0;
				a[i+1]=0;
				cnt++;
			}
		}
	}
	if(cnt==0 || cnt==1){
		cout << 0 << endl;
	}else{
		cout << ans << endl;
	}
	return 0;
}