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 <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <string>
#include <cmath>
#include <cstdlib>
#include <ctime>
using namespace std;
typedef long long LL;

LL ans;
int i,n,flag;
LL a[100005];
bool cmp(LL x,LL y)
{
	return x>y;
}
int main()
{
	cin>>n;
	for(i=1;i<=n;i++)
		cin>>a[i];
	sort(a+1,a+1+n,cmp);
	ans=1;
	flag=0;
	for(i=2;i<=n;i++)
	{
		if(a[i]==a[i-1])
		{
			i+=1;
			ans*=a[i];
			flag++;
			if(flag==2)
			{
				cout<<ans<<endl;
				break;
			}
		}
	}
	return 0;
}