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<cstdio>
#include <cstring>
#include <iomanip>
#include <queue>
#include <map>

using namespace std;

map <int,int> ans;

int main(){
    priority_queue<int> res;
    int n;
    scanf("%d",&n);
    for (int i = 0;i < n;i++){
        int k;
        scanf("%d",&k);
        ans[k]++;
        if (ans[k] == 2){
            res.push(k);
        }
        else if(ans[k] == 4){
            res.push(k);
        }
    }
    long long result = 1;
    for (int  i = 0;i<2;i++){
        if (res.empty()){
            printf("0\n");
            return 0;
        }
        else{
            result = result*res.top();
            res.pop();
        }
    }
    printf("%lld\n",result);
    return 0;
}