In the first version, the first person adds their salary, modulo 10NB, so you don't get any information. In the second version, you are correct, with a sign er

In the first version, the first person adds their salary, modulo 10NB, so you don't get any information. In the second version, you are correct, with a sign error. You are presenting an _upper_ bound on your salary, which is probably not a serious problem. That said, you do need negative numbers to truly conceal all the information about the initiator's salary.