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 compare_int(const void *a,const void *b){
  return *(long*)a-*(long*)b;
}

int main(void){
  long N;
  scanf("%ld",&N);
  long A[N];
  long ans[2];
  long past = -1;
  int flag = 0;

  for(int i = 0;i < N;i++) scanf("%ld",&A[i]);
  qsort(A,N,sizeof(long),compare_int);

  for(int i = N-1;i >= 0;i--){
    if(past == A[i]){
      if(flag){
        ans[1] = A[i];
        printf("%ld\n",ans[0]*ans[1]);
        return 0;
      }else{
        ans[0] = A[i];
        flag = 1;
      }
      past = -1;
    }else{
      past = A[i];
    }
  }

  printf("0\n");
  return 0;
}