 #include<iostream>
 #include<stdio.h>
 #include<string>
 #include<math.h>
 #include<iomanip>
 #include<algorithm>
 #include<string.h>
 #include<cctype>
 #include<map>
 #include<set>
 #include<vector>
 #include<sstream>
 #include<stack>
 #include<queue>
 #include<deque>
 #include<functional>
 #include<utility>
 
 using namespace std;

 int main()
 {
    int n;
    while(cin>>n)
    {
      long long int sum;
      sum=n*(n+1)/2+1;             
    }
    cout<<sum<<endl;
   //while(1);
   return 0;
 }

 