## Printer Errors

In a factory a printer prints labels for boxes. For one kind of boxes the printer has to use colors which, for the sake of simplicity, are named with letters from `a` to `m`.

The colors used by the printer are recorded in a control string. For example a *"good"* control string would be `aaabbbbhaijjjm` meaning that the printer used three times color a, four times color b, one time color h then one time color a...

Sometimes there are problems: lack of colors, technical malfunction and a *"bad"* control string is produced e.g. `aaaxbbbbyyhwawiwjjjwwm` with letters not from `a` to `m`.

You have to write a function printer_error which given a string will return the error rate of the printer as a string representing a rational whose numerator is the number of errors and the denominator the length of the control string. Don't reduce this fraction to a simpler expression.

The string has a length greater or equal to one and contains only letters from `a` to `z`.

**Examples:**

```rust
s = "aaabbbbhaijjjm"
printer_error(s) => "0/14"

s = "aaaxbbbbyyhwawiwjjjwwm"
printer_error(s) => "8/22"
```