Show the police force shared by counties with location on the east and west.
SELECT Police_force FROM county_public_safety WHERE LOCATION  =  "East" INTERSECT SELECT Police_force FROM county_public_safety WHERE LOCATION  =  "West"
police force shared by counties with location on the east and west refers to selecting police forces from counties where location is 'East' and intersecting it with selecting police forces from counties where location is 'West';