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 <cstdio>
#include <algorithm>
static const int MAXN = 1e5 + 4;

int n;
int a[MAXN];

int main()
{
    scanf("%d", &n);
    for (int i = 0; i < n; ++i) scanf("%d", &a[i]);

    std::sort(a, a + n);
    int s1 = 0, s2 = 0;
    for (int i = 0; i < n - 1; ++i) if (a[i] == a[i + 1]) {
        s2 = s1; s1 = a[i];
        ++i;
    }

    printf("%lld\n", (long long)s1 * s2);
    return 0;
}