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)*h/2)
a,b,h= map(int,open(0).read().split())
print((a+b)*h//2)