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First solution in Creative category for Pearls in the Box by abc512
def checkio(marbles, step):
nn, nw, C = len(marbles), marbles.count('w'), lambda n, m: n*C(n-1, m-1)//m if m else 1
return round(sum((nw, nn-nw)[i%2]*C(step-1, i)*(nn-1)**(step-i-1) for i in range(step))/nn**step, 2)
#These "asserts" using only for self-checking and not necessary for auto-testing
if __name__ == '__main__':
assert checkio('bbw', 3) == 0.48, "1st example"
assert checkio('wwb', 3) == 0.52, "2nd example"
assert checkio('www', 3) == 0.56, "3rd example"
assert checkio('bbbb', 1) == 0, "4th example"
assert checkio('wwbb', 4) == 0.5, "5th example"
assert checkio('bwbwbwb', 5) == 0.48, "6th example"
March 2, 2018
