How to fix bottom margin to 2.5 cm and keep the page numbering in the middle

I write my thesis using latex, and I have a problem with fixing the bottom margin with 2.5 cm including the page numbering in the middle of the distance of 2.5 cm as in the figure below

I use this command

usepackage[left=3.5cm,right=2.5cm,top=2.5cm,includefoot,bottom=2.5cm]{geometry}  

and I add this

setlength{footskip}{1.0cm}

The resulting margin in the pdf file is not correct
any suggestions ??

documentclass{article}
usepackage[left=3.5cm,right=2.5cm,top=2.5cm,bottom=2.5cm,showframe]{geometry}
usepackage{lipsum}
begin{document}
Default footskip = thefootskip

Bottom of text area = thedimexpr paperheight-1in-topmargin-headheight-headsep-textheightrelax

2.5cm = thedimexpr 2.5cmrelax

lipsum[1-10]
end{document}