input: Develop a python function that computes the nth Bell number.
  assert bell_Number(2) == 2
  assert bell_Number(3) == 5
  assert bell_Number(4) == 15
output:     def bell_Number(n: int) -> int:
          bell = [[0 for _ in range(n+1)] for _ 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]