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: Wrong Answer)

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
typedef struct node *link;
struct node{int d;  link next;};

link setLink(link p, int dis){
  link x = malloc(sizeof(struct node));
  x->d=dis; x->next=p;
  return x;
}

void printLink(link p){
  printf("link: ");
  while(p!=NULL){
    printf("%d->",p->d);
    p=p->next;
  }
  printf(".\n");
}

int main(void){
  int N, i, j, c=0, n, min=0, max=0, t, f;
  link p, q, head;  
  head = malloc(sizeof(struct node));
  head->next=NULL; head->d=0;
  scanf("%d",&N);
  for(i=0;i<N;i++){
    scanf("%d",&n);
    if(n<=min) continue;
    p=head;  q=head->next;  f=0;
    while(q!=NULL){

      if(q->d<n){
	p->next=setLink(p->next,n);  f=1;  break;
      }

      if(q->d==n){
	p->next=q->next;
	min = n;
	if(max<min){
	  t = min; min = max; max = t;
	}
	f=1;  break;
      }
      p=q;  q=q->next;
    }

    if(f==0){
      p->next=setLink(NULL,n);
    }
    //printf("%d: %d %d\n",i,max,min);
    //printLink(head);
  }
  printf("%d\n",max*min);
  return 0;
}