You are given a trapezoid. The lengths of its upper base, lower base, and height are a, b, and h, respectively.    An example of a trapezoid  Find the area of this trapezoid.
a, b, h=map(int, open(0).read().split())
print ((a+b)/2*h)
a, b, h=map(int, open(0).read().split())
print (int((a+b)/2*h))