Northwestern University:

The undergraduates have a number of traditions: Painting The Rock (originally a fountain donated by the Class of 1902) is a way to advertise, for example, campus organizations, events in Greek life, student groups, and university-wide events. Dance Marathon, a 30-hour philanthropic event, has raised more than 13 million dollars in its history for various children's charities. Primal Scream is held at 9 p.m. on the Sunday before finals week every quarter; students lean out of windows or gather in courtyards and scream. Armadillo Day, or, more popularly, Dillo Day, a day of music and food, is held on Northwestern's Lakefill every Spring on the weekend after Memorial Day. And in one of the University's newer traditions, every year during freshman orientation, known as Wildcat Welcome, freshmen and transfer students pass through Weber Arch to the loud huzzahs of upperclassmen and the music of the University Marching Band.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". How much money has the traditional Southwestern Dance Marathon raised for children's charities?
unanswerable