the second year of the university somebody started a study on the romantic
relations between the students. The relation “romantically involved” is defined
between one girl and one boy. For the study reasons it is necessary to find
out the maximum set satisfying the condition: there are no two students in the
set who have been “romantically involved”. The result of the program is the
number of students in such a set.

The input contains several data sets in text format. Each data set represents
one set of subjects of the study, with the following description:

the number of students
the description of each student, in the following format
student_identifier: (number_of_romantic_relations) student_identifier1 student_identifier2 ...
or
student_identifier: (0)

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