If the difference condition is that two numbers differ by one: first, neither Party A nor Party B is 1 or 9, otherwise the other party should know that it is 2 or 8. When Party A asks the question for the second time, he still doesn't know, which means that neither is 2 or 8. Similarly, 3 and 7 can be excluded for the third time, and only 4, 5 and 6 are left at this time. If Party A says it knows, it can only be 5 (whether Party B is 4 or 6, Party A is 5).