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 <cstdio>
#include <algorithm>

using namespace std;

int v[100010];

int main()
{
    //freopen("file.in","r",stdin);
    //freopen("file.out","w",stdout);
    int n,a=0,b=0;
    scanf("%d",&n);
    for(int i=1;i<=n;i++) scanf("%d",&v[i]);
    sort(v+1,v+n+1);
    int poz=n;
    while(poz>=1)
    {
        if(v[poz]==v[poz-1] && a==0) {a=v[poz];poz-=2;}
        else if(v[poz]==v[poz-1]) {b=v[poz];break;}
        else poz--;
    }
    printf("%lld",1LL*a*b);
    return 0;
}