using System;
    using System.Globalization;

    namespace a_b
    {
        public class Program
        {
            private static void Main()
            {
                int first = 0;
                int N = int.Parse(Console.ReadLine());
                if (N <= System.Math.Abs(1000))
                {
                    if (N == 1)
                    {
                        first = 1;
                        for (int i = 1; i <= N; i++)
                        {
                            first += i;
                        }
                    }

                    if (N > 1)
                    {
                        first = 0;
                        for (int i = 0; i <= N; i++)
                        {
                            first += i;
                        }
                    }

                    if (N < 1)
                    {
                        first = 1;
                        for (int i = 0; i >= N; i--)
                            first += i;
                    }
                    Console.WriteLine(first);
                }
            }
        }
    }