Calculate the total number of students in universities located in Sweden.
located in Sweden refers to country_name = 'Sweden'; number of students refers to num_students
SELECT SUM(T2.num_students) FROM university AS T1 INNER JOIN university_year AS T2 ON T1.id = T2.university_id INNER JOIN country AS T3 ON T3.id = T1.country_id WHERE T3.country_name = 'Sweden'