PGP-encrypted mail is welcome. Use my PGP public key.
For information about PGP, or a copy of the program itself, see my Introduction to PGP. GnuPG, available for all major operating systems, is also compatible with PGP. If you use RISC OS computers, the best solution is to use the RISC OS version of GnuPG.
Try this simple puzzle to test your mathematical IQ. The solution is immediately obvious once you recognise it, but some people with no mathematical training never get it!