The picture shows the velocity of a trolley on a straight track, made of straight lines. It moves forward, then reverses, and then keeps reversing at a steady speed. What is the total distance it travels in the $7$ seconds?

Fig. 1: A velocity-time graph with time t in seconds on the horizontal axis from 0 to 7 and velocity v in metres per second on the vertical axis. A blue broken line runs through the points (0, 0), (2, 4), (5, -2) and (7, -2). The part above the axis from t = 0 to t = 4 is shaded light blue, and the part below the axis from t = 4 to t = 7 is shaded light red.