19. Given that $n$ is a positive integer and $S=1+2+3+\cdots+n$. The units digit of $S$ cannot be some numbers. Find the sum of these numbers.
19. Ans: 22 .
$S=1+2+3+\cdots+n=\frac{n(n+1)}{2}$. The units digit of $S$ can only be $0,1,3,5,6,8$. Thus $S$ cannot end with $2,4,7$ or 9 . The sum is 22 .