decrease the value in label when i click button.

Question

Hi!
  i am taking one label, one Textbox, one button. in my label text is 150.
my aim i enter text in textbox and click button, that label shows a value is 149. and again i enter text in textbox and click button also decrease one value from label i.e., this time 148. similarly.
How to Write the code to do it. please can u provide a code todo this.
Thanks.