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<iostream>
#include<algorithm>
#include<vector>
#include<cmath>
#include<utility>
#include<cstdio>
#include<set>
#include<string>
#include<map>
#include<queue>
#include<stack>
#include <bitset>
using namespace std;
const int mod=1e9+7;
#define rep(i,n) for(int i=0;i<n;i++)
#define rep2(i,x,n) for (int i= x;i<n;i++)
#define all(v) v.begin(), v.end()
#define nepe(v) next_permutation(all(v))
#define F first 
#define S second
#define PB push_back 
#define MP make_pair
using ll = long long;
using vi = vector<int>; 
using vvi = vector<vi>;
using vp =vector<pair<int,int>>;

typedef pair<int,int> P;
string s;
int k;
ll INF = 10000000000000000;

int main(){
    int N;
    cin >>N;
    vector<ll> A(N);
    rep(i,N) cin >>A[i];
    sort(all(A));
    ll end1 = 0 ,end2 = 0;
    ll end = A[0];
    int count;
    rep2(i,1,N){
        if(A[i]==end){
            count ++;
            if(count >=4){
                end2 =end;
                end1 = end;
            }
            if(end!=end1){
                end2 = end1;
                end1 = end;
            }
        }
        else count = 1;
        end = A[i];
    }
    cout <<end1*end2<<endl;
}