Inside a rectangular room, measuring 30 feet in length and 12 feet in width and height, a spider is at a point on the middle of one of the end walls, 1 foot from the ceiling, as at A; and a fly is on the opposite wall, 1 foot from the floor in the centre, as shown at B.

What is the shortest distance that the spider must crawl in order to reach the fly, which remains stationary? 
Of course the spider never drops or uses its web, but crawls fairly.

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