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<iostream>
#include<vector>
#include<algorithm>
#include<queue>
using namespace std;

int gcd(int a,int b){
    if(a%b==0){
        return b;
    }
    else{
        return gcd(b,a%b);
    }
}

int main(){
    int n,h=0,w=0;
    cin>>n;
    vector<int>a(200000);

    for(int i=0;i<n;i++){
        cin>>a[i];
    }
    sort(a.begin(),a.end(),greater<>());
    for(int i=0;i<n-1;i++){
        if(h==0&&a[i]==a[i+1]){
            h=a[i];
            i++;
        }
        else if(a[i]==a[i+1]){
            w=a[i];
            break;
        }
    }
    cout<<h*w<<endl;
    return 0;
}