Based on my small expertise in this subject,
the answer should be 1/(20)^22
the logic is the same as a coin flip
1st flip: call for heads, 1/2 chance
2nd: call for heads, (multiply by self) 1/2 x 1/2= 1/4 chance
3rd: heads, 1/4 x 1/2= 1/8
and so on...