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++ (GCC 9.2.1) (judge verdict: Wrong Answer)

#include <bits/stdc++.h>
#include <algorithm>
#include <map>
#include <cstdlib>
#include <cmath>
#define _GLIBCXX_DEBUG
#define rep(i,n) for(int i=0;i<n;i++)
using namespace std;
typedef long long ll;
const double pi=acos(-1);

ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
ll lcm(ll a,ll b){return a/gcd(a,b)*b;}

int main(){
    int n;
    cin >> n;
    vector<int> A(n);
    for(int i=0;i<n;i++) cin >> A[i];
    sort(A.begin(),A.end());
    int h=0,v=0;
    for(int i=n-1;i>0;i--){
        if(h==0){
            if(A[i]==A[i-1]){
                h=A[i];
                i--;
            }
        }else{
            if(A[i]==A[i-1]){
                v=A[i];
                break;
            }
        }
    }
    cout << h*v << endl;
}