input: Write a function to debug the given Python code by adding return statements to the functions and printing the result of f2(2).
  
  def f1(x):
      return x + 1
  def f2(x):
      return f1(x) * f1(x)
  
  print(f2(2))
  
  assert f2(2) == 9
  assert f2(0) == 1
  assert f2(-1) == 0
output: def f1(x):
      return x + 1
  
  def f2(x):
      return f1(x) * f1(x)