Tram loop express

Plan the most profitable morning run of a tram on a circular line, when it may run express through one stop.

The line is a loop of n stops numbered 0 to n - 1 in the order the trams travel; after stop n - 1 comes stop 0 again. The string boardings has one digit per stop: boardings[i] is how many passengers (in tens) board at stop i. Every stop at which the tram actually stops costs stop_cost (the same for every stop), so serving stop i earns int(boardings[i]) - stop_cost, which can be negative.

A run starts at any stop and then visits consecutive stops in travel order. It never visits a stop twice, so it covers at most n stops and may cross from stop n - 1 back to stop 0. During the run the driver may run express through at most one stop: that stop is passed without stopping, earning nothing and costing nothing. Every other stop of the run is served. A run must serve at least one stop.

Find the largest total earnings any run can achieve.

Input: boardings, a string of n digits (n >= 1); stop_cost, an integer between 0 and 9.

Output: the maximum total earnings, as an integer (it is negative when every stop loses money, because the run still has to serve one stop).