Digit Sum

Solutions are the following;

Try to pair these numbers... (0, 999), (1, 998), (2, 997),..., (499, 500). There are 500 pairs of numbers. Each pair of number add up 27. As an example... (0, 999) has digit sum of 27.

Since there are 500 pairs, the result follows.

500 $\times$ 27 = 13500

Add 1 to 13500 since the last number is 1000.

Hence the answer is $\boxed{13501}$ .