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>

/* ソート関数 */
int int_sort( const void * a , const void * b ) {
  /* 引数はvoid*型と規定されているのでint型にcastする */
  if( *( int * )a < *( int * )b ) {
    return 1;
  }
  else
  if( *( int * )a == *( int * )b ) {
    return 0;
  }
  return -1;
}

int main(void){
    int N;
    scanf("%d",&N);
    int a[N];
    for(int i=0;i<N;i++){
        scanf("%d",&a[i]);
    }
    qsort((void*)a,N,sizeof(a[0]),int_sort);
    int ans1=0;
    int ans2=0;
    for(int i=1;i<N;i++){
        if(a[i]==a[i-1]&&ans1==0)ans1=a[i];
        else if(a[i]==a[i-1]&&a[i]!=ans1)ans2=a[i];
        else if(a[i]==a[i-1]&&a[i]==ans1&&i!=N-1&&a[i+1]==ans1)ans2=ans1;
        if(ans2!=0)break;
    }
    long long int po=ans1*ans2;
    printf("%lld",po);
    return 0;
}