input: Write a function to generate a Fibonacci sequence of the first n numbers. If n is 1, return 0. If n is 2, return 1. Otherwise, return the sum of the two preceding numbers in the sequence.
  assert Fibonacci(1)==0
  assert Fibonacci(2)==1
  assert Fibonacci(3)==1
  assert Fibonacci(4)==2
  assert Fibonacci(5)==3
output: def Fibonacci(n):
      if n == 1:
          return 0
      elif n == 2:
          return 1
      else:
          a, b = 0, 1
          for _ in range(3, n + 1):
              a, b = b, a + b
          return b