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 yassin
 */

#include <fstream>

#include <iostream>
#include <vector>
#include <string>
#include <utility>
#include <algorithm>
#include <map>
#include <set>
#include <cmath>
#include <cstdlib>
#include <tuple>
#include <queue>
#include <functional>
#include <stack>
#include <numeric>
#include <cassert>

using namespace std;

class arc81C {
public:
    void solve(istream& in, ostream& out) {
        int N;
        in >> N;
        vector<long long> A(N);
        map<long long, int> cnt;
        for (int i = 0; i < N; ++i) {
            in >> A[i];
            ++cnt[A[i]];
        }
        sort(A.begin(), A.end(), greater<long long>());
        A.erase(unique(A.begin(), A.end()), A.end());
        long long res = 0;
        long long a = -1;
        for (int i = 0; i < N; ++i) {
            if (a == -1 && cnt[A[i]] >= 4) {
                out << A[i] * A[i] << "\n";
                return;
            } else if (cnt[A[i]] >= 2) {
                if (a == -1) {
                    a = A[i];
                } else {
                    out << a * A[i] << "\n";
                    return;
                }
            }
        }
        out << res << "\n";
    }
};


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

	arc81C solver;
	std::istream& in(std::cin);
	std::ostream& out(std::cout);
	solver.solve(in, out);
	return 0;
}