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<stdio.h>
#include<string.h>
#include<algorithm>
#include<stdlib.h>
#include<iostream>
using namespace std;
int a[100010];
int b[2];
long long x,y;
int main(void)
{
	int i,j,k,n,m;
	scanf("%d",&n);
	for(i=0;i<n;i++)
	scanf("%d",&a[i]);
	sort(a,a+n);
	k=0;
	for(i=n-2;i>=0;i--)
	{
		if(k==2)
		break;
		if(a[i]==a[i+1]&&a[i]!=b[k-1])
		{
			b[k]=a[i];
			k++;
		}
	}
	m=0;
	j=-1;
	for(i=n-2;i>=0;i--)
	{
		if(a[i]==a[i+1])
		{
			m++;
			if(m==3)
			{
				j=a[i];
				break;
			}
		}
		else
		m=0;
	}
	x=0;
	y=0;
	if(j!=-1)
	{
		x=j*j;
	}
	if(k==2)
	{
		y=b[0]*b[1];
	}
	if(x<y)
	x=y;
	printf("%lld\n",x);
	return 0;
}