var k,i,j,t,s:longint;
begin
  //assign(input,'coin.in');
  //assign(output,'coin.out');
  //reset(input);
  //rewrite(output);
  read(k);
  t:=1;
  j:=1;
  s:=0;
  for i:=1 to k do
    begin
      if i>j then
        begin
          inc(t);
          inc(j,t);
        end;
      inc(s,t);
    end;
  write(s);
  //close(input);
  //close(output);
end.

/**************************************************************
	Problem: 2340
	User: admin
	Language: Pascal
	Result: Wrong Answer
****************************************************************/