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: Accepted)

#include<bits/stdc++.h>
using namespace std;
int n,a[100010],h[100010],tot;
map<int,int> mp;
int main()
{
	scanf("%d",&n);
	for(int i=1;i<=n;i++)
	{
		scanf("%d",&a[i]);
		mp[a[i]]++;
	}
	sort(a+1,a+n+1);
	n=unique(a+1,a+n+1)-(a+1);
	for(int i=1;i<=n;i++)
	{
		if(mp[a[i]]>=2) h[++tot]=a[i];
		if(mp[a[i]]>=4) h[++tot]=a[i];
	}
	sort(h+1,h+tot+1);
	long long ans=(long long)h[tot]*h[tot-1];
	cout<<ans<<endl;
	return 0;
}