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 (GCC 5.4.1) (judge verdict: Accepted)

#include <stdio.h>
#include <stdlib.h>

int comp(const void *a, const void *b){
    return *(long int *)b - *(long int *)a;
}

int main(void){
    long int N;
    long int A[100000];
    int i, j;
    int flag;
    long int a, b;
    a = 0;
    b = 0;

    scanf("%ld", &N);
    for (i = 0; i < N; i++){
        scanf("%ld", &A[i]);
    }

    qsort(A, N, sizeof(long int), comp);

    flag = 0;

    for (i = 0; i < N; i++){
        if (A[i] == A[i + 1] && flag == 0){
            a = A[i];
            i++;
            flag = 1;
        }else if (A[i] == A[i + 1] && flag == 1){
            b = A[i];
            break;
        }
    }

    printf("%ld\n", a * b);
    
    return 0;
}