The problems in a problem set are arranged according to the lexicographical order of their code names. However, the program committee wants to get a fixed order of the problems. For example, the easiest problem should be put on the first place so that all the teams will be able to find it, or the letterD can be assigned to a really Difficult problem. Therefore, to obtain some predefined order of the problems in the problem set, the program committee needs to carefully choose the code names. This is just what you have to do.
，这一次是一个告别，或者一个永远的告别，
