title: deleteCharactersInRange work something wrong
body: <p>HI all! I am using message deleteCharactersInRange from NSMutableString. And there is a problem that this finction deletes range in a wrong way.
  Here is a sample of code that works wrong:</p>
  
  <p>-(void) btnClick
  {
  NSRange deleteRange = NSMakeRange(0, 1);
  [valueStr deleteCharactersInRange:deleteRange];
  [self ShowNumber];
  }</p>
  
  <p>I have a mutable string: "-21.256" and when I press the button btnClick it must delete "-" from the begining but it does it only after the 5th presses time. Ealier it worked fine, but now no. Help please, or what can I use instead this function? Thanx!</p>