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>

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

int main(void){
  int n,i,co=0;
  long long int ans,t[2];
  scanf("%d",&n);
  long long int a[n];
  for(i=0;i<n;i++) scanf("%lld",&a[i]);
  qsort(a,n,sizeof(long long int),cmp);
  for(i=0;i<n-1;i++){
    if(a[i]==a[i+1]){
      t[co]=a[i];
      co++;
      i++;
    }
    if(co==2){
      printf("%lld\n",t[0]*t[1]);
      return 0;
    }
  }
  printf("0\n");
  return 0;
}