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 <bits/stdc++.h>

using namespace std;

typedef long long LL;
typedef unsigned long long ULL;
typedef pair<int, int> PII;

const int N = 100010;
const double eps = 1e-6;

int n;
map<int, int> h;

int main()
{
    cin >> n;
    for(int i = 0; i < n; i++)
    {
        int x;
        cin >> x;
        h[x]++;
    }

    int w = 0;
    for(map<int, int>::iterator it = h.begin(); it != h.end(); it++)
        if(it->second > 1 && it->first > w)
            w = it->first;

    for(map<int, int>::iterator it = h.begin(); it != h.end(); it++)
        if(w == it->first)
            it->second -= 2;

    int t = 0;
    for(map<int, int>::iterator it = h.begin(); it != h.end(); it++)
        if(it->second > 1 && it->first > t)
            t = it->first;

    cout << 1LL * t * w << endl;
    return 0;
}