input: Write a function to debug the given Python code where the function `f1` has been improperly defined. Ensure that the function `f1` returns the correct value and that the function `f2` correctly uses `f1` to compute the desired result. The final output should print the result of calling `f2(2)`.
  
  def f1(x):
      return x + 1
  
  def f2(x):
      return f1(x) * f1(x)
  
  assert f2(2)==9
  assert f2(0)==1
  assert f2(-1)==0
  assert f2(3)==16
  assert f2(5)==36
output: def f1(x):
      return x + 1
  
  def f2(x):
      return f1(x) * f1(x)