1. Arrange the natural numbers $1,2,3, \cdots, 999$. 1.5 into the number $N=1234 \cdots 998999$. Then, the sum of the digits of $N$ is $\qquad$
II. 1.13500.
Since $1.2, \cdots .999$ contains 999 numbers. |1. $1+998=2+997$ 999 , for sure, hence the sum of each pair is 27. There are 500 pairs. Therefore, the sum of the digits of $N$ is $500 \times 27=13500$.