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;

}