using System; using System.Collections.Generic; using System.Text; using System.Linq; class Program { public void Proc() { Reader.IsDebug = false; ulong num = ulong.Parse(Reader.ReadLine()); ulong ans = 0; ulong hanb = num / 2; ulong src = num * (num + 1) / 2; if(num % 2 > 0) { src -= num; } src -= hanb; src = src / 2; if(num % 2 > 0) { src += num; } Console.WriteLine(src); } public class Reader { public static bool IsDebug = true; private static String PlainInput = @" 3 "; private static System.IO.StringReader Sr = null; public static string ReadLine() { if (IsDebug) { if (Sr == null) { Sr = new System.IO.StringReader(PlainInput.Trim()); } return Sr.ReadLine(); } else { return Console.ReadLine(); } } public static int[] GetInt(char delimiter = ' ', bool trim = false) { string inptStr = ReadLine(); if (trim) { inptStr = inptStr.Trim(); } string[] inpt = inptStr.Split(delimiter); int[] ret = new int[inpt.Length]; for (int i = 0; i < inpt.Length; i++) { ret[i] = int.Parse(inpt[i]); } return ret; } } static void Main() { Program prg = new Program(); prg.Proc(); } }