FromOneToN

var
  a: int64;

begin
  readln(a);
  if(a > 0) then
    if a mod 2 = 0 then
      writeln((a + 1) * (a / 2))
    else
      writeln((a + 1) / 2 * a)
  else
    if a mod 2 = 0 then
      writeln((a + 1) * ((2 - a) / 2))
    else
      writeln((a + 1) / 2 * (2 - a));
end.