First off, do you have the program working at the moment? (I.e. a page, which the user submits numbers, one at a time, and the numbers are stored in the database - even if they are wrong
If so, then its pretty simple, at the point before you save it, check it! If its wrong, dont save it and update the output rendered to the page (you could use a number of controls here, Literal, Label, Image, whatever). Then simply set a boolean flag to say "warning shown", if you receive the next number, then save it no matter what?
This is all really "rough round the edges" but its the general idea..