sicp-all-tasks/sicp/3_002e30

Exercise 3.30: Figure 3.27 shows a

ripple-carry adder formed by stringing together 
  n
 full-adders.
This is the simplest form of parallel adder for adding two 
  n
-bit binary
numbers.  The inputs 
  
    A
    1
  
, 
  
    A
    2
  
, 
  
    A
    3
  
, …, 
  
    A
    n
  
 and 

  
    B
    1
  
, 
  
    B
    2
  
, 
  
    B
    3
  
,
…, 
  
    B
    n
  
 are the two binary numbers to be added (each 
  
    A
    k
  
 and

  
    B
    k
  
 is a 0 or a 1).  The circuit generates 
  
    S
    1
  
, 
  
    S
    2
  
, 

  
    S
    3
  
, …, 
  
    S
    n
  
,
the 
  n
 bits of the sum, and 
  C
, the carry from the addition.  Write a
procedure ripple-carry-adder that generates this circuit.  The procedure
should take as arguments three lists of 
  n
 wires each—the 
  
    A
    k
  
, the

  
    B
    k
  
, and the 
  
    S
    k
  
—and also another wire 
  C
.  The major drawback of the
ripple-carry adder is the need to wait for the carry signals to propagate.
What is the delay needed to obtain the complete output from an 
  n
-bit
ripple-carry adder, expressed in terms of the delays for and-gates, or-gates,
and inverters?