sky.py

def prob(a, n, t):
  rest = t - a
  if rest < 2:
    return 0
  if rest == 2:
    return 4
  if rest <= n + 1:
    return rest + 1
  if rest <= 2 * n:
    return 2 * n - rest + 1
  return 0

def answer(n, t):
  sum = 0
  for i in range(n):
    a = i + 1
    if a == 1:
      sum += 2 * prob(a, n, t)
    else:
      sum += prob(a, n, t)
  return sum / (n + 1) / (n + 1) / (n + 1)