input: Write a function to debug the following Python code which includes two functions f1 and f2. The function f1 lacks a return statement, causing an issue when used within f2. Ensure both functions are correctly defined, and add the necessary return statement in f1 to make the code functional. After debugging, call f2 with an input and print the result.
  
  
  def f1(x):
      return x + 1
  
  def f2(x):
      return f1(x) * f1(x)
  
  print(f2(2)) # Output: 9
  assert f2(2) == 9
  
output: def f1(x):
      return x + 1
  
  def f2(x):
      return f1(x) * f1(x)