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<stdio.h>
#include<cstring>
#include<cmath>
#include<algorithm>
#include<map>
#define ll long long
using namespace std;
int main(){
	int n;
	cin>>n;
	ll a[n];
	int j=0;
	map<ll,ll> m;
	ll num;
	for(int i=0;i<n;i++){
		cin>>num;
		m[num]++;
	}
	ll flag=0;
    for(map<ll,ll>::reverse_iterator rit=m.rbegin();rit!=m.rend();rit++){
        if(rit->second>=2){
        	a[j++]=rit->first;
        	if(j>=2){
                break;
        	}
		}
        if((*rit).second>=4){
            flag=(*rit).first;
        }
        //cout<<(*rit).first<<","<<(*rit).second<<endl;
    }
	/*map<ll, ll>::iterator  iter;
	for(iter = m.end(); iter != m.begin(); iter--){

		cout<<iter->first<<" "<<iter->second<<endl;
    }
    for(iter = m.begin(); iter != m.end(); iter++){
        if(iter->second>=2){
        	a[j++]=iter->first;
		}
		cout<<iter->first<<" "<<iter->second<<endl;
    }
*/
    /*for(int i=0;i<j;i++){
    	cout<<a[i]<<" ";
	}*/

	if(flag){
        cout<<flag*flag<<endl;
    }else{
        if(j<2){
            if(flag){
                cout<<flag*flag<<endl;
            }else{
                cout<<0<<endl;
            }

        }else{
            cout<<a[j-1]*a[j-2]<<endl;
        }
    }
	return 0;
}