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)

/**
 * code generated by JHelper
 * More info: https://github.com/AlexeyDmitriev/JHelper
 * @author tommy6073
 */

#include <iostream>
#include <string>
#include <map>

using namespace std;

class CMakeARectangle {
public:
    void solve(std::istream& in, std::ostream& out) {
        long long N;
        in >> N;
        map<long long, long long> sides;
        for (int i = 0; i < N; ++i) {
            long long side;
            in >> side;
            ++sides[side];
        }
        long long pairs = 0;
        map<long long, long long>::reverse_iterator it;
        long long max;
        long long max2;
        for (it = sides.rbegin(); it != sides.rend(); ++it) {
            if (it->second >= 4 && pairs == 0) {
                pairs = 2;
                max = it->first;
                max2 = it->first;
                break;
            } else if (it->second >= 2) {
                if (pairs == 0) {
                    max = it->first;
                    pairs = 1;
                } else {
                    max2 = it->first;
                    pairs = 2;
                    break;
                }
            }
        }
        long long ret = 0;
        if (pairs == 2) {
            ret = max * max2;
        }
        out << ret;
    }
};


int main() {
	CMakeARectangle solver;
	std::istream& in(std::cin);
	std::ostream& out(std::cout);
	solver.solve(in, out);
	return 0;
}