#include <stdio.h>
#include <math.h>

int main()
{
	int n,a[500],i,j,count = 0;
	scanf("%d",&n);
	for(i = 0; i < n; i++)
		scanf("%d",&a[i]);
	i = 0;
	while(i < n)
	{
		j = i+1;
		while(j < n)
		{
			if(fabs(a[i]) == fabs(a[j]) && a[i]+a[j] == 0)
			{
				count++;
				break;
			}
			j++;
		}
		i++;
	}
	printf("%d",count);
	
	return 0;
}