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 cmp(const void*a,const void*b){
	return *(int*)b-*(int*)a;
}
int main(void){
	int N;
	scanf("%d",&N);
	int i;
	int A[N];
	int count = 0;
	long long a=0,b=0;
	for(i=0;i<N;i++)scanf("%d",&A[i]);
	qsort(A,N,sizeof(int),cmp);
	for(i=0;i<N;i++){
		count =0;
		while(A[i]==A[i+1]){
			i++;
			count++;
			if(count>0)break;
		}
		if(count>0){
			if(a==0)a = A[i];
			else{
				b = A[i];
				printf("%lld",a*b);
				return 0;
			}
		}
	}
	printf("0");
	return 0;
}