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 <cmath>
#include <map>
#include <vector>
#include <set>
#include <utility>
#include <cstdio>
#include <cstring>
#include <numeric>
using namespace std;
typedef long long ll;
#define rep(i, n) for (int i = 0; i < n; i++)
#define rrep(i, n, m) for (int i = n; i >= m; i--)

int main()
{
	int n, tmp, max[2] = {0};
	set<int> s, s2;
	rep(i, 4) s2.insert(0);

	scanf("%d", &n);
	rep(i,n) {
		scanf("%d", &tmp);
		if(s.count(tmp) == 1){
			cout << "inserts2 " << tmp << endl;
			s2.insert(tmp);
		}else{
			cout << "inserts1 " << tmp << endl;
			s.insert(tmp);
		}
	}

	cout << *prev(s2.end(), 1) * *prev(s2.end(), 2);
}