\ifthenelse{\isundefined{\letterpapertrue}}{
\geometry{a5paper,hmargin=10mm,tmargin=10mm,bmargin=4mm}
}{
\geometry{a5paper,hmargin=20mm,tmargin=10mm,bmargin=4mm}
}
\ifthenelse{\isundefined{\letterpapertrue}}{
\geometry{a5paper,hmargin=10mm,tmargin=10mm,bmargin=4mm}
}{
\geometry{a5paper,hmargin=20mm,tmargin=10mm,bmargin=4mm}
}