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<iostream>
#include<cstdio>
#include<algorithm>
#include<map> 
using namespace std;
map<int,int> stk;
struct node
{
	int l,sum;
	friend bool operator <(node t1,node t2)
	{
		return t1.l>t2.l;
	}
}s[100005];
int n,i,j,k,a[100005],top;
long long ans=1,tot;
int main()
{
	scanf("%d",&n);
	for(i=1;i<=n;i++) 
	{	scanf("%d",&a[i]);
 		if(!stk[a[i]]) stk[a[i]]=++top,s[stk[a[i]]].l=a[i];
 		s[stk[a[i]]].sum++;
	}
	sort(s+1,s+1+top);
	for(i=1;i<=n;i++) 
	{   
		if(s[i].sum>=2)
		{
			ans*=(long long)s[i].l;
			tot++;
			if(s[i].sum>=4 && tot<2) tot++,ans*=(long long)s[i].l;
		}
		if(tot==2) break;
	}
	if(tot!=2) ans=0;
	cout<<ans<<endl;
	return 0;
}