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)

#define _CRT_SECURE_NO_WARNINGS
#define __STDC_FORMAT_MACROS
#include <cinttypes>
#include <algorithm>
#include <cstdio>
#include <functional>
#include <vector>

using namespace std;

template<class T> void gc(T &x)
{
    char c;
    int s = 0;
    x = 0;

    c = getchar_unlocked();

    if (c == '-') s = 1;
    else if ('0' <= c && c <= '9')  x = c - '0';

    while (true)
    {
        c = getchar_unlocked();

        if (c < '0' || c > '9') break;
        x = x * 10 + (c - '0');
    }

    if (s) x = -x;
}

int main()
{
    int N;

    gc(N);

    vector<int> A;

    for (int i = 0; i < N; i++)
    {
        int tmp;
        gc(tmp);
        A.push_back(tmp);
    }

    sort(A.begin(), A.end(), greater<int>());

    int bk = -1;
    int64_t first = -1;
    int64_t second = -1;
    for (int i = 0; i < N; i++)
    {
        if (bk != A[i]) bk = A[i];
        else
        {
            if (first == -1)
            {
                first = bk;
                bk = -1;
            }
            else if(second == -1)
            {
                second = bk;
                break;
            }
        }
    }

    printf("%" PRId64 "\n",
        (first == -1) || (second == -1) ? 0 : first * second);
    return 0;
}