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;
#define pb push_back
#define ll int
#define mod 1000000007
#define F first
#define S second
#define mk make_pair
#define B(v) v.begin(), v.end()
ll t;
int main()
{	
	int n;
	cin>>n;
	vector<ll>v;
	map<ll,ll>a;
	for(int i=0;i<n;i++)
	{
		cin>>t;
		a[t]++;
		if(a[t]==2)
		{
			v.pb(t);
			a[t]=0;
		}
	}
	sort(v.begin(),v.end());
	ll l=v.size();
	if(v.size()>=2)
	{
		cout<<v[l-1]*v[l-2]<<endl;
	}
	else
	{
		cout<<0<<endl;
	}
}