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>
#define ll          long long
#define pb          push_back
#define mp          make_pair
#define pii         pair<int,int>
#define vi          vector<int>
#define all(a)      (a).begin(),(a).end()
#define sz(x)       (int)x.size()
#define hell        1000000007
#define lbnd        lower_bound
#define ubnd        upper_bound
#define bs          binary_search
using namespace std;

int main()
{
	int n;
	cin>>n;
	int a[100005];
	map<long long int,long long int> m;
	set<long long int>b,c;
	vector<long long int>e,d;
	for(int i=1;i<=n;i++)
	{
		cin>>a[i];
		if(m.find(a[i])==m.end())
		m.insert(mp(a[i],1));
		else
		m[a[i]]++;
	}
    map<long long int,long long int>::iterator it2;
	for(it2=m.begin();it2!=m.end();it2++)
	{
		if(it2->second>=2)
		b.insert(it2->first);
		if(it2->second>=4)
		c.insert(it2->first);
	}
	
    set<long long int>::iterator it,it1;
    for(it=b.begin();it!=b.end();it++)
    {
    	d.pb(*it);
	}
	for(it1=c.begin();it1!=c.end();it1++)
    {
    	e.pb(*it1);
	}
	
	
	if(d.size()<2&&e.size()==0)
	cout<<0;
	else if(d.size()<2&&e.size()>0)
	cout<<e[e.size()-1]*e[e.size()-1];
	else if(d.size()>1&&e.size()==0)
	cout<<d[d.size()-1]*d[d.size()-2];
	else if(e.size()>0&&d.size()>1)
	cout<<max(e[e.size()-1]*e[e.size()-1],d[d.size()-1]*d[d.size()-2]);
	
	
}