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 <algorithm>
#include <set>
#include <map>

using namespace std;
using ll = long long int;

int main()
{
    int n; cin >> n;
    vector<int> a(n);
    map<int, int> m;
    for (int i = 0; i < n; ++i) cin >> a[i];
    set<int> s(a.begin(), a.end());
    ll ans;
    int h = 0, w = 0;

    for (int i = 0; i < n; ++i) {
        m[a[i]] += 1;
    }

    for (auto rit = s.rbegin(); rit != s.rend(); ++rit) {
        int num = *rit;
        int tmp = m[num];
        if (tmp >= 4 && h == 0 && w == 0) {
            ans = (ll)num * num;
            break;
        } else if (tmp >= 2) {
            if (h != 0) {
                w = num;
                ans = (ll)h * w;
                break;
            } else {
                h = num;
                tmp = 1;
            }
        } 
    }

    if (h == 0 || w == 0) ans = 0LL;
    cout << ans << endl;
        
    return 0;
}