input: Write a function to generate a Fibonacci sequence of the first n numbers. The function should return the nth Fibonacci number. 
  
  def Fibonacci(n):
      if n==1:
          return 0
      elif n==2:
          return 1
      else:
          return Fibonacci(n-1)+Fibonacci(n-2)
  
  assert Fibonacci(1)==0
  assert Fibonacci(2)==1
  assert Fibonacci(5)==3
  assert Fibonacci(10)==34
  assert Fibonacci(20)==4181
output: def Fibonacci(n):
      if n==1:
          return 0
      elif n==2:
          return 1
      else:
          return Fibonacci(n-1)+Fibonacci(n-2)