#include <iostream>
#include <set>
using namespace std;
int arr[10005];
int a[100005];
int main(){
    int n;
    cin>>n;
    set<int> s;
    for (int i=0;i<n;i++){
        int a1;
        cin>>a1;
        s.insert(a1);
    }
    int c=0;
    for (int i:s){
        for (int j:s){
            for (int z:s){
                if (j==z){
                    continue;
                }
                else if (j+z==i){
                    if ((arr[j]==0 || arr[z]==0) && a[i]==0){
                        arr[j]+=1;
                        arr[z]+=1;
                        a[i]+=1;
                        c+=1;
                        //cout<<j<<" "<<z<<" "<<i<<" "<<z+j<<endl;
                    }
                }
            }
        }
    }
    cout<<c;
    return 0;
}