What is the student population of the university that scored 98 in 2013?
student population refers to num_students; in 2013 refers to year = 2013
SELECT SUM(T1.num_students) FROM university_year AS T1 INNER JOIN university_ranking_year AS T2 ON T1.university_id = T2.university_id WHERE T2.score = 98 AND T1.year = 2013