import re

def checkio(marbles, step):

b=marbles.count("b")

n=len(marbles)    

for i in range(1,step+1):

    if i==1:

        c="(b-0)" 

    else:

        pat='b'+(i-1)*'.\w'

        s=re.findall(pat,c)

        for i in s:                

            ti='('+i+')'+'*('+i+'-1)'+'+(n-('+i+'))*('+i+'+1)'

            c=c.replace(i,ti)

            #print(c)

d=1-eval(c)/(n**step) 

d=round(d,2)

print(d)

return d

if __name__ == '__main__':

assert checkio("wwwwwwwwwwwwwwwwwwww",14) == 0.64, "1st example"