program Sum (input, output);
  var lI, lN : longint;
  begin
    readln (lN);
    if lN > 0 then lI := (lN + 1) * lN div 2;
    if lN = 0 then lI := 1;
    if lN < 0 then lI := (abs (lN) + 1) * lN div 2 + 1;
    writeln (lI);
  end.