#include <bits/stdc++.h>
using namespace std;
int main(void) {
    std::deque<int> deq;
    for (int i=0;i<10001;i++) {
        if (i==0) {
            deq.emplace_back(0);
        } else if (i==1) {
            deq.emplace_back(2);
        } else {
            deq.emplace_back(deq[i-1]+i);
        }
    }
    int n;
    while (cin>>n) {
        cout<<deq[n]<<endl;
    }
}
