from random import randint, sample

s = []
for i in xrange(100000):
  r = randint(1,81)
  a = ([True] * r) + ([False] * (81 - r))
  b = sample(a, 9)
  if b[0] and b.count(True) == 3:
    s.append(r)

print float(sum(s)) / len(s)