input: Write a function to print the Fibonacci series up to n numbers where n is user input. If n is less than or equal to 0, print "Invalid input".
  assert fibonacci_series(10)=="Fibonacci Series:\n0 1 1 2 3 5 8 13 21 34 "
  assert fibonacci_series(5)=="Fibonacci Series:\n0 1 1 2 3 "
  assert fibonacci_series(0)=="Invalid input"
  assert fibonacci_series(-5)=="Invalid input"
  assert fibonacci_series(1)=="Fibonacci Series:\n0 "
output: def fibonacci_series(n):
      if n <= 0:
          return "Invalid input"
      result = "Fibonacci Series:\n"
      a, b = 0, 1
      for _ in range(n):
          result += str(a) + " "
          a, b = b, a + b
      return result