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 <algorithm>
#include <cstdio>
#include <iostream>
#include <numeric>
#include <string>
#include <vector>

using namespace std;

int main() {
    int N;
    cin >> N;
    vector<int> As;
    int A;
    for (int i = 0; i < N; i++) {
        cin >> A;
        As.push_back(A);
    }
    sort(As.begin(), As.end());
    int h = 0, w = 0;

    for (int i = N - 1; i > 0; --i) {
        if (As[i] == As[i - 1]) {
            if (h == 0) {
                h = As[i];
                --i;
            } else {
                w = As[i];
                break;
            }
        }
    }
    cout << (long long int)h * w << endl;

    return 0;
}