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 <bits/stdc++.h>

using namespace std;

using ll = long long;

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

    map<int, int> memo;
    int n, a;
    cin >> n;
    for (int i = 0; i < n; ++i) {
        cin >> a;
        ++memo[a];
    }

    ll l1 = -1, l2 = -1;
    for (const auto &it : memo) {
        if (2 <= it.second) {
            if (l2 == -1 || l2 < it.first) l2 = it.first;
            if (l1 < l2) swap(l1, l2);
        }
        if (4 <= it.second) {
            if (l2 == -1 || l2 < it.first) l2 = it.first;
            if (l1 < l2) swap(l1, l2);
        }
    }

    if (l2 == -1) cout << "0\n";
    else cout << l1 * l2 << endl;

    return 0;
}