The widths (in meters) of a kidney-shaped swimming pool were measured at 7-meter intervals as indicated in the figure. Use the Midpoint Rule with n = 4 to estimate the area S of the pool if a1 = 3.1, a2 = 3.6, a3 = 3.4, a4 = 2.8, a5 = 2.5, a6 = 2.4, and a7 = 2.4.