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: 
[Wolfram Language (Mathematica)](https://www.wolfram.com/wolframscript/), 42 bytes
==================================================================================

```
FromDigits[9-IntegerDigits@--n]~Sum~{n,#}&

```

[Try it online!](https://tio.run/##y00syUjNTSzJTE78n277360oP9clMz2zpDjaUtczryQ1PbUIwnfQ1c2LrQsuza2rztNRrlX7H1CUmVeioO@Qru9QbaBjqGMIJAzAGEIY1P7/DwA "Wolfram Language (Mathematica) – Try It Online")

-2 bytes from att
[J-uby](https://github.com/cyoce/J-uby), 32 bytes
=================================================

```
:*|:sum+(-[S|:+@|:**&10,:~]|+:+)

```

[Attempt This Online!](https://ato.pxeger.com/run?1=m700K740qXLBIjfbpaUlaboWNxWstGqsiktztTV0o4NrrLQdaqy0tNQMDXSs6mJrtK20NaHqNAoU3KINYrlAlCGUgnENEAyDWIj6BQsgNAA)

Explanation
-----------

```
:* | :sum + ( -[ S | :+@ | :** & 10, :~ ] | +:+ )

:* |                                               # 0...input
     :sum + (                                   )  # Sum by
              -[                   ,    ] | +:+    #   Sum of
                 S | :+@ | :** & 10                #   10 to the power of the number of digits
                                     :~            #   and two's complement

```