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++14 (GCC 5.4.1) (judge verdict: Accepted)

#include<iostream>
#include<stdio.h>
#include<string.h>
#include<algorithm>
using namespace std;
long long int  a[100500];
int flag[100500];
bool cmp(int a,int b){
    return a>b;
}
int main(){
    int n;
    cin>>n;
    for(int i=0;i<n;i++){
        scanf("%d",a+i);
    }
    memset(flag,0,sizeof(flag));
    sort(a,a+n,cmp);
    long long int  x,y;
    x=y=0;
    int fg=0;
    for(int i=0;i<n-1;i++){
        if(a[i]==a[i+1]&&!flag[i]){
            if(!x) x=a[i];
            else{
                y=a[i];
                break;
            }
            flag[i]=1;
            flag[i+1]=1;
        }
    }
    long long int  sum;
    sum=x*y;
    cout<<sum<<endl;
    return 0;
}