p03997 AtCoder Beginner Contest 045 - Trapezoids
You are given a trapezoid. The lengths of its upper base, lower base, and height are a, b, and h, respectively.

<image>

An example of a trapezoid

Find the area of this trapezoid.

Constraints

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

Input

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


a
b
h


Output

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

Examples

Input

3
4
2


Output

7


Input

4
4
4


Output

16
```python
a,b,h=[int(input()) for i in range(3)];print(int((a+b)*h/2))
```