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 9.2.1) (judge verdict: Accepted)

#include<stdio.h>

void quick(long *,int,int);


int main(){
  long X=0,Y=0,ans=0;
  int N;
  scanf("%d",&N);
  long A[N];
  for (int i = 0; i < N; i++)
  {
    scanf("%ld",&A[i]);
  }
  
  quick(A,0,N-1);

  for (int i = N-1; i > 0; i--)
  {
    if (A[i]==A[i-1])
    {
      int j=0;
      while (A[i]==A[i-1]){i--;j++;}
      if (j>2&&X==0)
      {
        ans=A[i]*A[i];
        break;
      }else{
        if (X>0)
        {
          Y=A[i];
          ans=X*Y;
          break;
        }else{
          X=A[i];
        }
      }
    }
  }
  
  printf("%ld",ans);

  return 0;
}

void quick(long a[],int left,int right)
{
    int s,t,i,j;

    if (left<right){
        s=a[left];            
        i=left; j=right+1;      
        while (1){              
            while (a[++i]<s);
            while (a[--j]>s);
            if (i>=j) break;
            t=a[i]; a[i]=a[j]; a[j]=t;
        }
        a[left]=a[j]; a[j]=s;

        quick(a,left,j-1);
        quick(a,j+1,right);
    }
}