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;

using ll = long long;
using vi = vector<int>;
using vll = vector<long long>;

int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};

constexpr long long mod = 1000000007;

#define rep(i, n) for (int i = 0; i < n; i++)

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

    int n;
    cin >> n;
    vll a(n);
    rep(i, n) cin >> a[i];

    sort(a.begin(), a.end(), greater<ll>());

    ll w = -1, h = -1;
    if (n >= 4) {
        for (int i = 0; i < n - 1; i++) {
            if (w < 0) {
                if (a[i] == a[i + 1]) {
                    w = a[i];
                }
                i++;
            } else {
                if (a[i] == a[i + 1]) {
                    h = a[i];
                    break;
                }
            }
        }
    }

    if (h > 0 && w > 0) {
        cout << w * h << endl;
    } else {
        cout << 0 << endl;
    }
}