Two buildings are standing opposite to each other on either side of the road. From a point on the road, ladder reaches building's window which is 12 m high from the ground. Keeping the foot of the ladder at the same point, the ladder is turned on the other side of the road and it reaches a window which is 9 m high from the ground. If the length of the ladder is 15 m, then the width of the road (in m) is :