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 <vector>
#include <numeric>//accumulate(v.begin(), v.end(), 0);
#include <algorithm>//sort(v.begin(),v.end());
#include <string>//
using namespace std;
#define ll long long
#define rep(i, n) for(int i = 0; i < (int)(n); i++)

int main(){
        int n;
        cin >> n;
        vector <int> v(n,0);
        rep(i,n){
                cin  >> v[i];
        }
        sort(v.begin(),v.end());
        int a=0,b;
        int i=n-1;
        int w=0,h=0;
        while(true) {
                if(v[i]==v[i-1] &&w==0) {
                        w=v[i];
                        a++;
                        if(a==3) {
                                h=v[i];
                                break;
                        }
                }else if(v[i]==v[i-1]) {
                        h=v[i];
                        break;
                }else{
                        a=0;
                }
                i--;
                if(i==0) {
                        break;
                }
        }
        cout << h*w << endl;

        return 0;
}