class Main {
    
    public static void main(String[] args) {
        
        int n = Integer.parseInt(args[0]);
        
        String outStr = String.valueOf(sum(1, n));
        
        System.out.println(outStr);
        
    }
    
    public static int sum(int n1, int n2) {
        int result = (n1 + n2) * n2 / 2;
        return result;
    }
    
}