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<cstdio>
#include<iostream>
#include<cstring>
#include<algorithm>
#include<vector>
using namespace std;
long long int sum;
int a[100006],biaoji,s[100006];
int main()
{
    int n,q,tt;
    while(scanf("%d",&n)==1)
    {
        biaoji=0;sum=1;
        memset(s,0,sizeof(s));
        for(int i=1;i<=n;i++)
        {
            scanf("%d",&a[i]);
        }
        sort(a+1,a+n+1);
        tt=0;
        s[1]=1;
        for(int i=2;i<=n;i++)
        {
            if(a[i]==a[i-1])
            {
                s[i]=s[i-1]+1;
            }
            else
                s[i]=1;
        }
        q=0;
        for(int i=n;i>=1;i--)
        {
            if(s[i]>=4)
            {
                sum=a[i]*a[i];
                biaoji=1;
                break;
            }
            if((s[i]==3||s[i]==2)&&sum!=a[i])
            {
                sum=sum*a[i];
                q++;
                if(q>=2)
                {
                    biaoji=1;
                    break;
                }
            }

        }
        if(biaoji==1)printf("%lld\n",sum);
        if(biaoji==0)printf("0\n");
    }




}