Mr Ravi enters a square office building (30 m x 30 m) through the east-facing door in the middle of the east wall. He walks 15 m south to the stairs in the central lobby and climbs to the top floor. The building has one staircase in the central lobby and four lifts, one in each corner; an outer corridor joins the lifts to the entrance on the ground floor. If Ravi takes a lift instead, roughly how far must he walk from the door?