ubstruction, especially binary may look for you much more difficult but do not worry just remember that substractin 0 does not change anything so 0-0=0; 1-0=0.
If you substruct 1 from 0 it gives you even -1 or if there is a higher binary position you have to substruct from it and it gives you 1. After this operation that higher position has to be reduced by so if it was 1 you get 0, If there was 0 on that position you have to get to the next higher position and so on.
Examples:
10-1=1; 101-1=100; 110-1=101; 1000-1=111, etc.
You may write -1 above the position affected so you will know it was reduced.
You will master this after having done several exercises in the window above.
Enjoy !
Witold W.