Apply CF After every 20 row

hi,
i want to change background color after 20 rows using conditional formatting

for example
Range a10:a29 = red background color
Range a30:a49 = blue background color
Range a50:a69 = red background color
Range a70:a89 = blue background color
Range a90:a109 = red background color
Range a110:a129 = blue background color and so on

thanks in advance

thank you very much