#19807: 終於寫出來了~~
eric104510@gmail.com
(Deus)
#include <iostream>
using namespace std;
int main(){
int n;
while(cin>>n)
cout<<1+n*(n-1)/2<<endl;
}
#include <iostream>
using namespace std;
int main(){
int n;
while(cin>>n)
cout<<1+n*(n-1)/2<<endl;
}