A special electronic sensor is embedded in the seat of a car that takes riders around a circular loop-the-loop at an amusement park. The sensor measures the magnitude of the normal force that the seat exerts on a rider. The loop-the-loop ride is in the vertical plane and its radius is 19 m. Sitting on the seat before the ride starts, a rider is level and stationary, and the electronic sensor reads 660 N. At the top of the loop, the rider is upside-down and moving, and the sensor reads 340 N. What is the speed of the rider at the top of the loop?