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<bits/stdc++.h>
using namespace std;
int main()
{

map<long long int,long long int>m;
vector<long long int>v;
long long int a,b,c,d,r=0,i,j=0,k,l;
cin>>a;
for(i=1;i<=a;i++)
{
 cin>>b;
        m[b]++;
        if(m[b]==1)v.push_back(b);
}
    sort(v.begin(),v.end());
    //cout<<v.size()<<endl;
for(i=v.size()-1;i>=0;i--)
{

  if(m[v[i]]>=2)
  {
   r++;
  if(r==1)k=v[i];
            if(r==1&&m[v[i]]>=4)
            {
            j=1;c=v[i];
        }
  if(r==2)l=v[i];m[v[i]]=0;
  }
}
  if(j==1)
   {
   cout<<c*c<<endl;
   }
    else
    {
    if(r>1)cout<<l*k<<endl;
else cout<<"0\n";}
}