input: Write a python function to generate the nth Fibonacci number.
  assert fibonacci_series(10) == 34
  assert fibonacci_series(5) == 3
  assert fibonacci_series(7) == 8
output: def fibonacci_series(n):
      if n <= 0:
          return 0
      elif n == 1:
          return 1
      else:
          a, b = 0, 1
          for _ in range(n-1):
              a, b = b, a + b
          return a