Contest: Task: Related: TaskB

Score : $100$ points

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.

- $1≦a≦100$
- $1≦b≦100$
- $1≦h≦100$
- All input values are integers.
- $h$ is even.

The input is given from Standard Input in the following format:

$a$ $b$ $h$

Print the area of the given trapezoid. It is guaranteed that the area is an integer.

3 4 2

7

When the lengths of the upper base, lower base, and height are $3$, $4$, and $2$, respectively, the area of the trapezoid is $(3+4)×2/2 = 7$.

4 4 4

16

In this case, a parallelogram is given, which is also a trapezoid.