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;
typedef long long ll;
map<ll,ll>mp;
const int N=1e5+7;
ll a[N];
int main()
{
	ll n;
	cin>>n;
	for(int i=1;i<=n;i++)
	{
		cin>>a[i];
		mp[a[i]]++;
	}
	sort(a+1,a+1+n);
	ll flag=0;
	ll ans1=0,ans2=0;
	ll sum1=0;
	for(int i=n;i>=1;i--)
	{
		if(mp[a[i]]>=4)
		sum1=max(sum1,a[i]*a[i]);
	}
	for(int i=n;i>=1;i--)
	{
		if(mp[a[i]]>=2)
		{
			if(flag&&a[i]!=ans1)
			{
				ans2=a[i];
				break;
			}
			else
			{
				flag=1;
				ans1=a[i];
			}
		}
	}
	ll sum2=ans1*ans2;
	if(sum1>sum2)
	cout<<sum1<<endl;
	else cout<<sum2<<endl;
}