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 <algorithm>
#include <functional>
#include <iostream>
#include <vector>
using namespace std;
#define REP(i, n) for (int i = 0; i < (int)(n); i++)
#define all(x) (x).begin(), (x).end()
typedef long long ll;

int main() {
  int N;
  cin >> N;
  vector<ll> A;
  REP(i, N) {
    int a;
    cin >> a;
    A.push_back(a);
  }
  sort(all(A));
  reverse(all(A));
  ll width = 0, length = 0;
  REP(i, N - 1) {
    if (A[i] == A[i + 1]) {
      if (width) {
        length = A[i];
        break;
      } else
        width = A[i];
      if (i == N - 1) break;
      i += 1;
    }
  }

  cout << width * length << endl;
  return 0;
}