Two trains, each having a speed of 21 km/h, are headed at each other on the same straight track. A bird that can fly 42 km/h flies off the front of one train when they are 42 km apart and heads directly for the other train. On reaching the other train it flies directly back to the first train, and so forth. (We have no idea why a bird would behave in this way.) What is the total distance (in km) the bird travels?