one me; one keyboard

We pick two numbers a and b, so that a>=b and both numbers are within the range [2,99]. We give Mr.P the product a*b and give Mr.S the sum
a+b.
The following dialog takes place:
Mr.P: I don’t know the numbers
Mr.S: I knew you didn’t know. I don’t know either.
Mr.P: Now I know the numbers
Mr.S: Now I know [...]