title: subtracting letters in c
body: <p><strong>I would like to know how to "subtract letters" in C:</strong><br>
  I mean, I have 2 letters, 'a' and 'c' and i want to execute 'c'-'a'='b' which is 3-1=2.<br>
  How is it possible to obtain the same behaviour in C?<br>
  I can conversion <em>Letters->Numbers</em> but how to manage the <em>limited lenght</em> of the alphabet? <strong>Thank you.</strong></p>