plain TeX: different margins for odd and even pages

In plain TeX, I set paper size and margins with, e.g.:

\hoffset=-1in
\voffset=-1in
\special{papersize=210mm,297mm}
\hsize=210mm
\vsize=297mm
\advance\hoffset by5mm
\advance\hsize by-10mm
\advance\voffset by10mm
\advance\vsize by-20mm

Now, suppose I want to set different margins for add and even pages, say 5mm
left and 10mm right for odd pages and the opposite for even pages: please
suggest whoever can a proper way to do it.

Thanks in advance,

Rodolfo