from random import choice

numtests = 1000000

def test():
    bacs = 1
    time = 0

    while time <> 30:
        newbacs = 0
        for b in range(bacs):
            newbacs += choice((0,1,2,2))
        bacs = newbacs
        time += 10
    if bacs == 6: return True


numsuccess = 0
for i in range(numtests):
    if test(): numsuccess = numsuccess + 1

print float(numsuccess)/numtests