Find minimum tanks to cover all houses

You are given a string `S` representing a street: - `'H'` = a house - `'-'` = an empty plot where you may place a water tank A house is considered **covered** (able to collect rainwater) if there is a water tank placed **adjacent** to it (either immediately to its left or right). You may place tanks only on empty plots (`'-'`). Your task is to compute the **minimum number of tanks** needed so that **every house** in `S` is covered. If it is impossible to cover all houses, return `-1`. Example: - Input: `S = "-H-HH--"` - One optimal placement is `"-HTHHT-"` (where `'T'` denotes a tank), which uses `2` tanks. Implement a function that returns the minimum number of tanks required (or `-1` if impossible).