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++ (GCC 9.2.1) (judge verdict: Accepted)

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cmath>
#include<vector>
#include<map>
#include<set>
#include<string>
#include<queue>
#include<stack>
#include<complex>
using namespace std;
#define MOD 1000000007
#define MOD2 998244353
#define INF (1<<29)
#define LINF (1LL<<60)
#define EPS (1e-10)
#define PI 3.1415926535897932384626433832795028
typedef long long Int;
typedef pair<Int, Int> P;
typedef long double Real;
typedef complex<Real> CP;


int n;
map<int ,int> cnt;
int a;
int main(){
    cin >> n;
    for(int i = 0;i < n;i++){
        cin >> a;
        cnt[a]++;
    }
    vector<Int> edge;
    for(auto it = cnt.rbegin();it != cnt.rend();it++){
        if(it->second >= 2){
            edge.push_back(it->first);
            it->second -= 2;
        }
        if(it->second >= 2){
            edge.push_back(it->first);
            it->second -= 2;
        }
    }
    if(edge.size() < 2)cout << 0 << endl;
    else cout << edge[0] * edge[1] << endl;
    return 0;
}