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 9.2.1) (judge verdict: Runtime Error)

#include"stdio.h"
int main(){
    int n;
    scanf("%d",&n);
    int a[n],max=0;
    for(int i=0;i<n;i++){
        scanf("%d",&a[i]);
        if(max<a[i])
            max=a[i];
    }
    int f[max+1];
    for(long long int i=0;i<max+1;i++)
        f[i]=0;
    for(int i=0;i<n;i++){
        f[a[i]]++;
    }
    long long int first=0,second=0;
    for(long long int i=max;i>=0;i--){
        if(first==0){
            if(f[i]>=4){
                printf("%lld\n",i*i);
                return 0;
            }else if(f[i]>=2){
                first=i;
            }
        }else{
            if(f[i]>=2){
                second=i;
                printf("%lld\n",first*second);
                return 0;
            }
        }
    }
    printf("0\n");
}