I attempted to solve this problem using c++, but I got WA #9, I want to ask a question that if there is no key to compute, how to output it,
such as input 1, 30. The first 26 CPUs should test one password, and the last 4 got nothing ,so how can I output it?

You've been assured that there'll be no input where M< the tot no of words possible. Did you write the program without using long arithmetic? Coz i program in c++ and cant think of a way to handle integers of the order of 10^50. Let me know if you know any method or have implemented any class to handle such numbers.