input: Write a python function to compute 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]