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>
int cmp(const void *a, const void *b)
{
    return *(int*)b-*(int*)a;
}
int s[100005];
int main()
{
    unsigned long long int a=0;
    unsigned long long int b=0;
    int flaga=0;
    int flagb=0;
    int n;
    unsigned long long int ans;
    scanf("%d",&n);
    for(int i=0;i<n;i++){
        scanf("%d",&s[i]);
    }
    qsort(s,n,sizeof(int),cmp);
    for(int i=0;i<n;){
        if(s[i]==s[i+1] && flaga==0){
            a=s[i];
            i+=2;
            flaga=1;
        }
        else if(s[i]==s[i+1] && flagb==0){
            b=s[i];
            i+=2;
            flagb=1;
        }
        else{
            i++;
        }
    }
    ans=a*b;
    printf("%llu\n",ans);
}