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>

namespace Task {

#define long long long

std::map<int, int> map;

inline void solve() {
    std::ios::sync_with_stdio(false), std::cin.tie(NULL), std::cout.tie(NULL);
    register int n, cnt = 2;
    register long ans = 1;
    std::cin >> n;
    for (register int i = 0, t; i < n; i++) std::cin >> t, map[t]++;
    for (auto it = map.rbegin(); it != map.rend(); it++) {
        if (it->second > 1) ans *= it->first, cnt--;
        if (cnt == 0) break;
    }
    if (cnt == 0)
        std::cout << ans;
    else
        std::cout << '0';
}

#undef long
}

int main() {
    Task::solve();
    return 0;
}