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 <map>
#include <string>
using namespace std;

int main(){
    int n;
    cin >> n;
    map<int,int> maps;
    long long ans=0;
    int m1=0;
    int m2=0;

    for(int i=0;i<n;i++){
        int a;
        cin>>a;
        if (maps.find(-a) != maps.end() ) {
            maps[-a]++;
        }else{
            maps[-a]=1;
        }
        if (maps[-a]>=2){
            if (m1<a){
                m1=max(m1,a);
                maps[-a]-=2;
            }else if (m2<a){
                m2=max(m2,a);
                maps[-a]-=2;
            }
        }
    }
    ans=m1*m2;
    cout <<ans<<endl ;
    return 0;
}