Instruct: 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?
Output: SELECT SUM USN 2013 FROM table WHERE BW 2013 < 1000 AND Forbes 2011 > 17 AND CNN 2011 < 13