
I believe in this case, the issue would be that the user has inputted a negative number instead of using an incorrect format. Perhaps it would be better to flag up to 'Input a quantity integer greater than 0!' so that the user can easily edit his input to work.
Also, although the instruction is to type help to see more, in the help for add function the quantity being an integer greater than zero should be specified.
I believe in this case, the issue would be that the user has inputted a negative number instead of using an incorrect format. Perhaps it would be better to flag up to 'Input a quantity integer greater than 0!' so that the user can easily edit his input to work.
Also, although the instruction is to type help to see more, in the help for add function the quantity being an integer greater than zero should be specified.