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>
long long int main(){
  int N,i,j=0,tmp,k,x,y,xi,yi;
  long long int M[2];
  M[0]=0;
  M[1]=0;
  scanf("%d",&N);
  int a[N];
  for(i=0;i<N;i++){
    scanf("%d",&a[i]);
  }
  x=0;y=0;
  k=0;
  i=0;
  while(k<N && j<2){
    for(i=0;i<N;i++){
      if(x<a[i]){
	y=x;
	yi=xi;
	x=a[i];
	xi=i;
      }else if(y<a[i]){
	y=a[i];
	yi=i;
      }
    }
    if(x==y){
	M[j]=x;
	j++;
	a[xi]=0;
	a[yi]=0;
    }else{
      a[xi]=0;
    }
    k++;
    x=0;
    y=0;
  }
  printf("%lld",M[0]*M[1]);
  return 0;
}