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;
typedef long long ll;
#define rep(i, a, b) for (int i = a; i < b; i++)
//'A' = 65, 'Z' = 90, 'a' = 97, 'z' = 122

int main() {
    int n;
    cin >> n;
    ll a[n];
    rep(i, 0, n) cin >> a[i];

    sort(a, a + n, greater<ll>());

    int cnt = 1;
    ll x = 0;
    ll y = 0;
    rep(i, 0, n - 1) {
        if (a[i] == a[i + 1]) cnt++;
        else {
            if (cnt == 4) {
                if (x) {
                    y = a[i];
                    break;
                } else {
                    x = a[i];
                    y = a[i];
                    break;
                }
            } else if (cnt == 2 || cnt == 3) {
                if (x) {
                    y = a[i];
                    break;
                } else x = a[i];
            }
            cnt = 1;
        }
    }

    cout << x * y << endl;
    return 0;
}