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 <iostream>
#include <string>
#include <map>
#include <vector>
#include <utility>
#include <set>
#include <stack>
#include <queue>
#include <deque>
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <numeric>

#define rep(i,j,k) for(int i=(int)j;i<(int)k;i++)
#define itrep(i,x) for(auto i=(x).begin(); i!=(x).end();i++)
#define Sort(x) sort((x).begin(),(x).end())
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define vec vector
#define INF (int)1e9
#define INFL 1e18
#define MOD 1000000007
#define pb push_back
#define MP make_pair
#define PI 3.1415926535
#define fcout cout<<fixed<<setprecision(10)
typedef long long int ll;
typedef std::pair<int,int> P;
int D=1;
int dx[4]={0,1,0,-1},dy[4]={-1,0,1,0};
int day[12]={31,28,31,30,31,30,31,31,30,31,30,31};
//template<typename T>void pp(T x){cout<<x<<endl;}

using namespace std;
template<typename T> void test(vector<T> v){for(int i=0;i<v.size();i++){if(i)cout<<" ";cout<<v[i];}cout<<endl;}

int main(){
    int n;
    cin>>n;
    vector<ll> v(n);
    rep(i,0,n)cin>>v[i];
    
    Sort(v);
    ll ans[2]={};
    ll spans=0;
    int index=0;
    rep(i,0,n){
        int ct=0;
        while(v[i]==v[i+1]){
            ct++;
            i++;
        }

        if(ct>=3)spans=v[i];
        else if(ct && ans[0]<v[i]){
            ans[1]=ans[0];
            ans[0]=v[i];
        }
    }
    cout<<max(spans*spans,ans[1]*ans[1-1])<<endl;
    

    return 0;
}