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 co(const void *a, const void *b) {
  return *(long long int *)b - *(long long int *)a;
}
int main(){
  long long int n,i,x[100010],tmp[3];
  scanf("%lld",&n);
  for(i=0;i<n;i++){
    scanf("%lld",&x[i]);
  }
  qsort(x,n,sizeof(long long int),co);
  tmp[0]=x[0];
  int l=0;
  for(i=1;i<n;i++){
    if(tmp[l]==x[i])l++;
    else tmp[l]=x[i];
    if(l==2)break;
  }
  if(l==2)printf("%lld\n",tmp[0]*tmp[1]);
  else puts("0");
  return 0;
}