8. When the positive integer $m$ satisfies what condition,
$$1+2+\cdots+(m-1)+m \equiv 0(\bmod m)$$

is always true (without calculating the left-hand sum)?
8. $1+2+\cdots+(m-1)+m \equiv m+(1+(m-1))+(2+(m-2))+\cdots \equiv$ $0(\bmod m)$, when $2 \nmid m ; \equiv m / 2 \not \equiv 0(\bmod m)$, when $2 \mid m$.