Question: 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](https://oeis.org/A061601) in the OEIS).Your task is to add the first \$n\$ elements.

### Input

A number \$n∈[0,10000]\$.

### Output

The sum of the first \$n\$ elements of the sequence.

### Test cases

```
0 -> 0
1 -> 9
10 -> 45
100 -> 4050
1000 -> 408600
10000 -> 40904100

```

Standard [code-golf](/questions/tagged/code-golf "show questions tagged 'code-golf'") rules apply. The shortest code in bytes wins.

Answer: 
[Dyalog APL](https://www.dyalog.com/), 17 bytes
===============================================

```
+/⍳-⍨1-⍨10*≢∘⍕¨∘⍳

```

`⎕IO←0`.

[Try it online!](https://tio.run/##SyzI0U2pTMzJT9dNzkksLs5M/v@ob6qn/6O2CQb/04Cktv6j3s26j3pXGIIJA61HnYsedcx41Dv10Aowvfn//7RDKwwUDBUMgYQBGEMIAwA)
[Python 2](https://docs.python.org/2/), 42 bytes
================================================

```python
f=lambda n:n and-~~n+10**len(`n-1`)+f(n-1)

```

[Try it online!](https://tio.run/##FcVNCoAgFEbReat4Q38StGHQSiLQMEmoT5EmTdy62eXCye9zJkytheVy9@4dYQY5eFUrpNFCXAeYhTKWy8C6vIVUKFIErXok0//RepsH6uUS8bDAIuftAw "Python 2 – Try It Online")