user: Question: How many millions of U.S. viewers whatched episodes written by Krystal Houghton?
Columns: ['No. in series', 'No. in season', 'Title', 'Directed by', 'Written by', 'Original air date', 'U.S. viewers (millions)']
Types: ['real', 'real', 'text', 'text', 'text', 'text', 'text']
Sample row: [73.0, 1.0, '"From the Grave"', 'Karen Gaviola', 'Ann Donahue & Elizabeth Devine', 'September19,2005', '19.21']
SQL:

assistant: SELECT SUM("U.S. viewers (millions)") FROM "Table" WHERE "Written by" = 'Krystal Houghton';