instruction: What is the USN 2013 ranking with a BW 2013 ranking less than 1000, a Forbes 2011 ranking larger than 17, and a CNN 2011 ranking less than 13?
response: SELECT SUM("USN 2013") FROM table_37643 WHERE "BW 2013" < '1000' AND "Forbes 2011" > '17' AND "CNN 2011" < '13'