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 <bits/stdc++.h>

using namespace std;

vector <int> d;
map<int, int> k;

int main()
{
    int n;
    scanf("%d", &n);
    d.resize(n);
    for (int i = 0; i < n; i++){
        scanf("%d", &d[i]);
        k[d[i]]++;
    }
    sort(d.begin(), d.end());
    int a1 = 0, a2 = 0;
    for (int i = n - 1; i >= 1; i--){
        if (d[i] == d[i - 1]){
            a1 = d[i];
            d[i] = 0;
            d[i - 1] = 0;
            break;
        }
    }
    sort(d.begin(), d.end());
    for (int i = n - 1; i >= 1; i--){
        if (d[i] == d[i - 1]){
            a2 = d[i];
            break;
        }
    }
    cout << a1 * a2;
    return 0;
}