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: Wrong Answer)

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

static int cmp_int(const void* v1, const void* v2)
{
  const long long _v1 = *((const long long*)v1);
  const long long _v2 = *((const long long*)v2);

  if ( _v1 > _v2 ) {
    return -1;
  } else if ( _v1 < _v2 ) {
    return 1;
  } else {
    return 0;
  }
}
int main(void)
{
  long long a[100002], m[2]={0};
  int n, i, j, c;
  c = 0; j = 0;
  scanf("%d", &n);

  for(i = 0; i < n; i++)
    scanf("%lld", &a[i]);

  qsort(a, n, sizeof(long long), cmp_int);
  
  i = 0;
  do{
    while(a[i] == a[i+1]){
      c++;
      i++;
    }
    if(c != 0){
      m[j] = a[i];
      j++;
      c = 0;
    }
    i++;
    if(i == n) break;
  }while(j != 2);

  printf("%lld\n", m[0]*m[1]);

  return 0;
}