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)

#include <iostream>
#include <string>
#include <vector>
#include <algorithm>

#define rep(i,n) for (int i = 0; i < (n); ++i)

using std::cin;
using std::cout;
using std::endl;
using std::string;
using std::vector;

int main() {
  int N;
  cin >> N;

  vector<int> a(N);
  rep(i, N) {
    cin >> a[i];
  }

  std::sort(a.begin(), a.end());

  int before = -1;
  int w = 0;
  int h = 0;
  for(int i = 0; i < N; i++) {
    if (before == a[i]) {
      w = h;
      h = a[i];
      before = -1;
    } else {
      before = a[i];
    }
  }

  cout << (long long)w * h << endl;

  return 0;
}