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 <vector>
#include <algorithm>

using namespace std;



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

    int n;
    cin >> n;

    vector<int> v;
    int x;

    for (int i = 0; i < n; ++i) {
        cin >> x;

        v.push_back(x);
    }

    sort(v.begin(), v.end());

    int l = 0, L = 0;

    for (vector<int>::reverse_iterator it = v.rbegin(); it != v.rend(); ++it) {
        if (*it == *(it + 1)) {
            if (L == 0) L = *it, ++it;
            else {
                l = *it;
                cout << L * 1LL * l;
                return 0;
            }
        }
    }

    cout << 0;

    return 0;
}