Conversation so far:
when did christina perri write a thousand years
Christina Perri saw the film Breaking Dawn Part 1 six months before it came out, and wrote 'A Thousand Years' for Edward and Bella.
when did the movie breaking dawn part 1 come out
The Twilight Saga Breaking Dawn Part 1 was released on November 18, 2011.

Question: how many copies did the song sell
Self-contained rewrite: how many copies did the song a thousand years sell