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: Wrong Answer)

#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
const int N=1e6+1;
int A[N],n,p,d;
LL rez;
bool cmp(int a,int b){
    return (a>b);
}
int main()
{
    scanf("%d",&n);
    for(int i=0;i<n;i++){
        scanf("%d",&A[i]);
    }
    sort(A+0,A+n,cmp);
    int i=0;
    while(i<n){
        if(A[i]==A[i+1]){
            if(p){
                d=A[i];
                LL x=(LL)p*A[i];
                cout<<x;
                return 0;
            }
            p=A[i];
            while(A[i]==A[i+1]){
                i++;
            }
        }
        i++;
    }
    LL x=(LL)p*d;
    cout<<x;
    return 0;
}