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>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
#include <math.h>

int sort(const void *a, const void *b){
    return *(int*)b-*(int*)a ;
}

int main(void){
    int n;
    scanf ("%d",&n);
    int a[n];
    bool choosen[n];
    for (int i=0; i<n; i++){
        scanf("%d",&a[i]);
        choosen[i]=false;
    }
    qsort (a,n,sizeof(int),sort);
    long long yoko=0,tate=0;
    for (int i=0; i<n-1; i++){
        if (a[i]==a[i+1] && choosen[i]==false && choosen[i+1]==false){
            yoko=a[i];
            choosen[i]=true;
            choosen[i+1]=true;
            break;
        }
    }
    for (int i=0; i<n-1; i++){
        if (a[i]==a[i+1] && choosen[i]==false && choosen[i+1]==false){
            tate=a[i];
            break;
        }
    }
    long long ans;
    ans=yoko*tate;
    printf("%lld\n",ans);
	return 0 ;
}