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>
#include <string.h>
#include <math.h>

int main(void){
  int N, i, j, A[100000], c=0, n, min=0, max=0, t;

  scanf("%d",&N);
  for(i=0;i<N;i++){
    scanf("%d",&n);
    if(n<=min) continue;
    for(j=0;j<c;j++){
      if(A[j]==n){
	A[j]=0;
	min = n;
	if(max<min){
	  t = min; min = max; max = t;
	}
	break;
      }
    }
    if(j==c){ A[c]=n; c++;}
  }
  printf("%d\n",max*min);
  return 0;
}