#include<iostream>
#include<string>
#include<vector>
#include<cstdio>
#include<sstream>
#include<algorithm>
#include<cmath>
#include<map>
#include<functional>
using namespace std;
int stoi(string x){stringstream ss;ss<<x;int tmp;ss>>tmp;return tmp;}
string itos(long double x){stringstream ss;ss<<x;return ss.str();}
int main(){
int n,ans=1;
while(cin>>n){
ans=1;
for(int i=0;i<n;i++)ans+=(i+1);
cout<<ans<<endl;
}
return 0;
}