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 main(){
	int i,j,n,m=0;
	scanf("%d",&n);
	if(n<4) return 0;
	int *p=(int*)malloc(n*sizeof(int));
	for(i=0;i<n;i++){
		scanf("%d",&p[i]);
	}
	for(i=0;i<n;i++){
        if(p[i]<1) return 0;
	}
    int (*A)[2]=(int(*)[2])malloc(n*2*sizeof(int));
	for(i=0;i<n;i++){
		for(j=0;j<m;j++){
			if(p[i]==A[j][0]){
				A[j][1]++;
				break;
			}
		}
		if(j==m){
			A[m][0]=p[i];
			A[m][1]=1;
			m++;
		}
	}
	int max1=0,max2=0;
	for(i=0;i<m;i++){
		if(A[i][1]>1&&A[i][0]>max1){
			max2=max1;
			max1=A[i][0];
		}
	}
	if(max1==0||max2==0){
		printf("0\n");
	}
	else printf("%d\n",max1*max2);
	return 0;
}