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>
#include <string>
#include <algorithm>
#include <string.h>
using namespace std;
 
int main(){

    int N;
    long long  a[100010],max=0,second=0;

    cin >> N;

    for(int i=0;i<N;i++){
        cin >> a[i];
    }

    sort(a,a+N);

    for(int i=0;i<N-1;i++){
        if(a[i]==a[i+1]){
            if(a[i]>max){
                second=max;
                max=a[i];
            }else if(a[i]>second){
                second=a[i];
            }
        }
    }

    cout << max * second << endl;
}