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;
using ll=long long int;
#define rep(i,n) for(int i=0;i<(n);i++)

int main(){
    int n;
    cin>>n;
    map<ll,ll>mpx;
    map<ll,ll>mpy;
    vector<int>a(n);
    rep(i,n){
        cin>>a[i];
    }
    rep(i,n){
        mpx[a[i]]++;
    }
    ll mx=0;
    ll my=0;
    for(auto p:mpx){
        if(p.second>=2){
            mx=max(mx,p.first);
        }
    }
    sort(a.begin(),a.end());
    int cnt=0;
    rep(i,n){
        if(a[i]==mx){
            a[i]=0;
            cnt+=1;
            if(cnt==2)break;
        }
    }
    rep(i,n){
        mpy[a[i]]++;
    }
    for(auto p:mpy){
        if(p.second>=2){
            my=max(my,p.first);
        }
    }
    
    cout<<mx*my<<endl;
}