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: Time Limit Exceeded)

#include <stdio.h>

int main(){

  int n,a[100000],i;
  int count[100000]={};

  scanf("%d",&n);
  for(i=0;i<n;i++){
    scanf("%d",&a[i]);
  }
  int j,k;
  for(i=0;i<n;i++){
    for(j=0;j<n-i;j++){
      if(a[j]<a[j+1]){
        k=a[j];
        a[j]=a[j+1];
        a[j+1]=k;
      }
    }
  }
  int s=0;
  for(i=1;i<n;i++){
    if(a[i-1]==a[i]){
      count[s++]=a[i];
      a[i]=0;
    }
    if(s>1)break;
  }

  if(s<2)printf("0\n");
  else printf("%d\n",count[0]*count[1]);

  return 0;
}