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: Wrong Answer)

#include <stdio.h>
#include <stdlib.h>
int compare_int(const void *a, const void *b)
{
    return *(int*)b - *(int*)a;
}

int main(){
	int n,a,b,c=0,i,data[100000]={0};
	scanf("%d",&n);
	for(i=0;i<n;i++){
		scanf("%d",&data[i]);
	}

	qsort(data, n, sizeof(int), compare_int);
	
	for(i=0;i<n;i++){
		if(i==0){
			a=data[i];
			
		}else{
			if(a==data[i]){
				c+=1;
				if(c==2){
					break;	
				}
				b=a;
				a=0;
			}else{
				a=data[i];
			}
		}
	}
	if(c==2){
		printf("%d\n",a*b);
	}else{
		printf("0\n");	
	}
	
return 0;
}