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++ (GCC 9.2.1) (judge verdict: Wrong Answer)

/*
  Contest 071
  C - Make a Rectangle
  Rakesh Kumar --> 09/09/2020
 */

#include <bits/stdc++.h>

int main() {
    std::ios_base::sync_with_stdio(false);
    std::cin.tie(0); std::cout.tie(0);

    int n = 0;
    std::cin >> n;
    std::map<int, int> dict;
    std::vector<int> r;
    while (n--) {
        int l = 0;
        std::cin >> l;
        ++dict[l];
        if (dict[l] == 2 || dict[l] == 4)
            r.emplace_back(l);
    }

    std::sort(r.begin(), r.end());
    if (r.size() < 2)
        std::cout << 0 << std::endl;
    else
        std::cout << r[r.size() - 1] * r[r.size() - 2] << std::endl;

    return 0;
}