include<bits/stdc++.h>
include
using namespace std; int n,b[150],c; set a;
int main(){
cin>>n;
for(int i=0;i<n;i++){
cin>>b[i];
a.insert(b[i]);
}
for(int i=0;i<n-1;i++){
for(int j=i+1;j<n;j++){
if(a.count(b[i]+b[j]))
c++;
a.erase(b[i]+b[j]);
}
}
cout<<c;
return 0;
}