input: Write a function to debug and fix the provided Python code, ensuring that the function `f1` returns a value, and the function `f2` uses the return value of `f1` correctly. The corrected code should produce the expected output when `f2(2)` is called.
  assert f2(2)==9
output: def f1(x):
      return x * x
  
  def f2(y):
      return f1(y) + 5