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 (GCC 5.4.1) (judge verdict: Accepted)

#include <stdio.h>
#include <stdlib.h>

int cmpnum(const void * n1, const void * n2)
{
	if (*(long *)n1 > *(long *)n2)
	{
		return -1;
	}
	else if (*(long *)n1 < *(long *)n2)
	{
		return 1;
	}
	else
	{
		return 0;
	}
}

int main(void) {

  long n;
  scanf("%ld", &n);
  long a[n];
  for (long i = 0; i < n; i++) {
    scanf("%ld", &a[i]);
  }
  qsort(a, n, sizeof(long), cmpnum);
  int pick = 0;
  long answer = 1;
  for (long i = 0; i < n; i++) {
    if (a[i+1] == a[i]) {
      answer *= a[i];
      pick += 2;
      i++;
    }
    if (pick == 4) {
      break;
    }
  }
  if (pick == 4) {
    printf("%ld\n", answer);
  } else {
    printf("0\n");
  }

  return 0;
}