Let's consider the following sequence:
$$9,8,7,6,5,4,3,2,1,0,89,88,87,86,85,84,83,82,81,80,79,78,77,76,75,74,73,72,71...$$ This is the sequence of \$9\$'s complement of a number: that is, \$ a(x) = 10^d - 1 - x \$ where \$ d \$ is the number of digits in \$ x \$. (A061601 in the OEIS).Your task is to add the first \$n\$ elements.
A number \$n∈[0,10000]\$.
The sum of the first \$n\$ elements of the sequence.
0 -> 0 1 -> 9 10 -> 45 100 -> 4050 1000 -> 408600 10000 -> 40904100
Standard code-golf rules apply. The shortest code in bytes wins.