The numbers can be grouped by pairs:

999,999,999 and 0;

999,999,998 and 1'

999,999,997 and 2;

and so on....

There are half a billion pairs, and the sum of the digits in each pair is 81. The digits in the unpaired number, 1,000,000,000, add to 1. Then:

(500,000,000 X 81) + 1= 40,500,000,001.