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 compare(const void *a, const void *b){
	long long int *A = (long long int *)a;
	long long int *B = (long long int *)b;
	if (*A > *B) return 1;
	if (*A < *B) return -1;
	return 0;
}


int main(){
	int N;
	scanf("%d",&N);
	long long A[N];
	for(int i=0;i<N;i++)scanf("%lld",&A[i]);
	qsort(A,N,sizeof(long long),compare);
	long long len[N];
	int c=0;
	for(int i=0;i<N-1;i++){
		if(A[i]==A[i+1]){
			len[c]=A[i];
			c++;
			i++;
		}
	}
	if(c<2)printf("0\n");
	else printf("%lld\n",len[c-1]*len[c-2]);
	return 0;
}