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: Runtime Error)

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


int NMAX = 1000000007;

int main(void){
    int n;
    scanf("%d", &n);

    int i, x;
    int *cnt = NULL;
    cnt = (int *)calloc(NMAX, sizeof(int));


    int m = 1;
    for(i = 0; i < n; ++i){
        scanf("%d", &x);
        cnt[x]++;
        m = x > m ? x : m;
    }

    int a = 0, b = 0;
    for(i = m; i >= 0; --i){
        if(cnt[i] >= 2){
            if(!b)
                b = i;
            else{
                a = i;
                break;
            }
        }
    }
    printf("%d\n", a * b);

    free(cnt);
    return 0;
}