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 <stdio.h>
#include <algorithm>
#include <iostream>

int main()
{
  long n;
  long array[100001];

  scanf("%ld", &n);

  for(long i = 0; i < n; i++)
    scanf("%ld", &array[i]);

  std::sort(array, array + n);
  std::reverse(array, array + n);

  long match[100001];
  long counter = 0;

  for(long i = 0; i < n; i++)
  {
    if(array[i] == array[i + 1])
    {
      match[counter] = array[i];
      counter++;
      i++;
    }
  }
  if(counter < 2)
    printf("%d\n", 0);

  else
    printf("%ld\n", match[0] * match[1]);
  return 0;
}