include <iostream>

using namespace std;
const int nm = 1e6 + 1;
int n, a[nm], i, dem, j;
int main () {
freopen("TONGBANGKHONG.INP","r",stdin);
freopen("TONGBANGKHONG.OUT","w",stdout);
cin >> n;
for (i = 0; i < n; i++)
cin >> a[i];
dem = 0;
for(i=0; i<n-1; i++)
for (j=i+1;j<n;j++)
if(a[i] + a[j] == 0)
dem++;
if (dem == 0)
cout << 0;
else
cout << dem << endl;
return 0;
}