#include <iostream>
#include <algorithm>
using namespace std;
#define N 500
int a[N]={0};
int main_2()
{
    int n,cnt=0;
    cin>>n;
    for (int i=0;i<n ;i++ )
    {
        cin>>a[i];
    }
    sort(a,a+n);
    int left=0,right=n-1;
    while (left<right&&a[left]<0&&a[right]>0)
    {
        if (-a[left]==a[right])
        {
            cnt++;
            left++;
            right--;
        }
        else if (-a[left]>a[right])
        {
            left++;
        }
        else
        {
            right--;
        }
    }
    cout << cnt << endl;
	return 0;
}