Help with Brain Teaser, please.?

You are locked in a 50 x 50 x 50 foot room which sits on 100 foot stilts. There are an open window at the corner of the room, near the floor with a strong hook cemented into the floor by the window. So if you had a 100 foot rope, you could tie one end to the hook, and climb down the rope to freedom. (The stilts are not accessible from the window.) There are two 50 foot lengths of rope, each cemented into the ceiling, about 1 foot apart, near the center of the ceiling. You are a strong, agile rope climber, good at tying knots, and you have a sharp knife. You have no other tools. The rope is strong enough to hold your weight, but not if it is cut lengthwise. You can survive a fall of no more than 10 feet. How do you get out alive?