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<math.h>
#include<string.h>
#include <time.h>
typedef long long llint;
typedef long double ldouble;
// 小大
int compare(const void *a, const void *b){return *(llint *)a - *(llint *)b;}
typedef struct{
    llint aa;
    llint bb;
}frequent;
// 小大
int cmp( const void *p, const void *q ) {
    return ((frequent*)p)->bb - ((frequent*)q)->bb;
}


int main(void){
    llint n,i,num1=0,num2=0;
    scanf("%lld",&n);
    llint a[n];
    for(i=0;i<n;i++){
        scanf("%lld",&a[i]);
    }
    qsort(a,n,sizeof(llint),compare);
    for(i=0;i<n-1;i++){
        if(a[n-1-i] == a[n-2-i]){
            if(num1!=0){
                num2=a[n-1-i];
                break;
            }else{
                num1=a[n-1-i];
            }
            i++;
        }
    }

    printf("%lld\n",num1*num2);
    return 0;
}