input: Write a python function to calculate the nth Bell number.
  assert bell_number(2) == 2
  assert bell_number(3) == 5
  assert bell_number(4) == 15
output: def bell_number(n):
      bell = [[0 for i in range(n + 1)] for j in range(n + 1)]
      bell[0][0] = 1
      for i in range(1, n + 1):
          bell[i][0] = bell[i - 1][i - 1]
          for j in range(1, i + 1):
              bell[i][j] = bell[i - 1][j - 1] + bell[i][j - 1]
      return bell[n][0]