-15.
3i * 5i
(Does that mean "iii * iiiii"?)
...
Silly but serious math digression here: If -15 is 3 * 5 * i^2, or 3 * 5 * i * i, or 3i * 5i, does that becomes i*i*i (3i) * i*i*i*i* (5i), or just (since i^1=1) 1?
... Err ... I used to be a math major. This used to be simple/trivial. What the heck am I doing wrong?
Answer: Drink coffee. 3i = i+i+i, not i*i*i. Silly me.