Puzzle of the Week #139 - Strange Sequence

I have a curious sequence: it begins: 2, 2, 2, 7, and then every subsequent number is the magnitude of the difference between the sum of the two previous numbers and the sum of the two numbers before that. Or in more mathematical language:

potw sequence.JPG

So, for instance the 5th number is 5, since (7+2)-(2+2) is 5. Remember since we are always looking for the magnitude of the difference, it cannot be negative.

What is the 123456789th number in the sequence?