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First solution in Clear category for Pearls in the Box by rp_shinya
def checkio(sequence, n):
r, w = len(sequence), sequence.count('w')
return 1/2 + (1-2/r)**(n-1) * (w/r-1/2)
#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"
Feb. 11, 2017