Distinct outage count

An uptime checker probes a site once a minute and stores the results in order as a list of strings, each "up" or "down". For the monthly report you need the number of distinct outages: an outage is a maximal run of consecutive "down" results — it starts when a "down" follows an "up" (or begins the list) and ends at the next "up" (or the end of the list). Return the number of outages; an empty list has zero.