\documentclass[10pt,a4paper,twoside]{article}
\usepackage[paperheight=18cm,paperwidth=10cm,top=5mm,bottom=20mm,right=5mm,left=5mm]{geometry}
%\usepackage[a4paper,top=2.5cm,bottom=2cm,left=2.5cm,right=2.5cm]{geometry}
\documentclass[10pt,a4paper,twoside]{article}
\usepackage[paperheight=18cm,paperwidth=10cm,top=5mm,bottom=20mm,right=5mm,left=5mm]{geometry}
%\usepackage[a4paper,top=2.5cm,bottom=2cm,left=2.5cm,right=2.5cm]{geometry}