It is giving some very strange answers to some very simple addition. I tried adding 1 to the numbers and that was strange:
1 + 1 = 1
1 + 2 = ERROR
1 + 3 = 3
1 + 4 = 4
1 + 5 = 9
1 + 6 = 8
1 + 7 = 7
1 + 8 = 8
1 + 9 = 9
So I tried a few more, also with strange results:
2 + 4 = 8
2 + 5 = 8
3 + 4 = 9
Why is it acting so strange?