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>

void quicksort(int a[], int l, int r){
  int s, t, i, j;
  if (l<r) {
    s=a[(l+r)/2]; i=l; j=r;
    while (1) {
      while (a[i]<s) i++;
      while (a[j]>s) j--;
      if (i>=j) break;
      t=a[i]; a[i]=a[j]; a[j]=t;
      i++; j--;
    }
    quicksort(a,l,i-1);
    quicksort(a,j+1,r);
  }
}

int main(){
  int n;
  scanf("%d",&n);

  int i,a[n];
  for(i=0;i<n;i++)scanf("%d",&a[i]);
  quicksort(a,0,n-1);
  int max=0,submax=0,k=0;
  for(i=n-1;i>=0;i--){
    k=a[i];
    //printf("%d ",k);
    if(a[i-1]==k){
      if(max==0){
        max=k;
        i--;
      }
      else{
        submax=k;
        break;
      }
    }

  }
  printf("%ld\n",(long)max*submax);
}