#include <iostream>
#include <cstdio>
#define MAX_N 10001
using namespace std;
int seq[MAX_N];
void init() {
    int ans = 1;
    for(int r = 0; r < MAX_N; r++) {
        seq[r] = ans; ans += r + 1;
    }
}

int main(void) {
    init();
    int input;
    while(cin >> input) {
        cout << seq[input] << endl;
    }
    return(0);
}