A programmer has a set of banknotes valued 1, 3, 9 ..., 3^k,... one each. He has to pay N in a restaurant (provided) He must pay that much M so: M can be paid with banknotes the programmer has, i.e. he cannot pay 5, because he does not have 2 bank notes with value of 1; and M-N must be represented the same way, i.e. if N is 4, M cannot be 9, because 5 cannot be represented as a sum of 3^K, one or less for each K. So if asked 4, a programmer must pay 13, 13=9+3+1, 13-4=9.

I hope you will understand this clumsy explanation.