How many trips started from Mountain View city and ended at Palo Alto city?
select count(*) from station as t1 join trip as t2 join station as t3 join trip as t4 on t1.id = t2.start_station_id and t2.id = t4.id and t3.id = t4.end_station_id where t1.city = "Mountain View" and t3.city = "Palo Alto"