#include <iostream>
using namespace std;
int n,a[110],s;
bool b[10010];
int main()
{
    cin>>n;
    for (int i=1;i<=n;i++)
        cin>>a[i];
    for (int i=1;i<=n;i++)
        for (int a1;a1<=n;a1++)
            for (int a2;a2<=n;a2++)
                if (a[a1]+a[a2]==a[i] && a1!=a2 && a1!=i && a2!=i && b[a[i]]!=1)
                {
                    s++;
                    a[i]=-999;
                    b[a[i]]=1;
                }
    cout<<s;
    return 0;
}