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 <algorithm>
#include <functional>
#include <map>
using namespace std;

map<int, int> c;
int arr[100001];

int main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);

    int n, a, x, y;
    cin >> n;

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

    sort(arr, arr + n, greater<int>());

    bool doneone = false, donetwo = false;

    for (int i = 0; i < n; ++i)
    {
        if (c[arr[i]] > 1 && !doneone)
        {
            x = arr[i];
            doneone = true;
        }
        else if (c[arr[i]] > 1 && !donetwo && arr[i] != x)
        {
            y = arr[i];
            donetwo = true;
            cout << x * y << '\n';
            return 0;
        }
    }
    cout << 0 << '\n';
}