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 <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <numeric>
#include <stdio.h>
#include <stack>
#include <queue>
#include <cstdio>
#include <cmath>
#include <iterator>
#include <map>
#include <fstream>
#include <cctype>

using namespace std;

#define Int int64_t

void printVec(std::vector<int>& vec) {
	std::cout << "";
	for (auto it = vec.begin(); it != vec.end(); ++it) {
		std::cout << *it << " ";
	}
	std::cout << std::endl;
}
void printVec1(std::vector<string>& vec) {
	std::cout << "";
	for (auto it = vec.begin(); it != vec.end(); ++it) {
		std::cout << *it << " ";
	}
	std::cout << std::endl;
}

template<typename T>
void remove(std::vector<T>& vector, unsigned int index)
{
	vector.erase(vector.begin() + index);
}

int vector_finder(std::vector<string> vec, string number) {
	auto itr = std::find(vec.begin(), vec.end(), number);
	size_t index = std::distance(vec.begin(), itr);
	if (index != vec.size()) { // 発見できたとき
		return 1;
	}
	else { // 発見できなかったとき
		return 0;
	}
}

#include <sstream> 
std::vector<std::string> StringSplit(const std::string& str, char sep)
{
	std::vector<std::string> v;
	std::stringstream ss(str);
	std::string buffer;
	while (std::getline(ss, buffer, sep)) {
		v.push_back(buffer);
	}
	return v;
}



int main() {
	int n; cin >> n;
	vector<int> a(n),b(n);
	for (int i = 0; i < n; i++) {
		cin >> a[i];
	}
	b = a;
	sort(a.begin(), a.end(), greater<int>());
	a.erase(unique(a.begin(), a.end()), a.end());

	int t1 = 0,four=0;
	size_t counts;

	for (int i = 0; i < a.size(); i++) {
		counts = count(b.begin(), b.end(), a[i]);
		//cout << a[i] << ":" << counts << endl;
		if (counts >= 4) {
			if (t1 != 0) {
				cout << t1 * a[i];
				break;
			}
			else {
				cout << a[i] * a[i];
				break;
			}
		}
		if (counts >= 2 && counts <= 3) {
			if (t1 == 0) t1 = a[i];
			else {
				cout << t1 * a[i];
				break;
			}
		}
	}
	if (t1 == 0 && four == 0) cout << 0;



	cout << endl;
	return 0;
}