import java.util.Arrays;
public class TwoSumFast
{
public static int count(int[] a)
{
Arrays.sort(a);
int N=a.length;
int cnt=0;
for(int i=0;i<N;i++)
if (BinarySearch.rank(-a[i],a)>i)
cnt++;
return cnt;
}
public static void main (String[] args)
{
int[] a=In.readInts(args[0]);
StdOut.println(count(a));
}
}
public class TwoSumFast
{
public static int count(int[] a)
{
Arrays.sort(a);
int N=a.length;
int cnt=0;
for(int i=0;i<N;i++)
if (BinarySearch.rank(-a[i],a)>i)
cnt++;
return cnt;
}
public static void main (String[] args)
{
int[] a=In.readInts(args[0]);
StdOut.println(count(a));
}
}