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

That means the input consists of only lowercase letters.

Based on the description: ... a "bad" control string is produced e.g. aaaxbbbbyyhwawiwjjjwwm with letters not from a to m.

Then a test case with input "aaabbbAAABBBcccCCC" should expect "9/18", and fits with the existing description.

Maybe the description should specify ... with lowercase letters not from a to m.?

Otherwise, a truly valid soltion should be able to handle uppercase letters being in the control string as well.

And invalidate almost 83k solutions? No, too late.