Problem Statement

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.

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.

Constraints

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

Sample Input 1

3
4
2

Sample Output 1

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.

Sample Input 2

4
4
4

Sample Output 2

16

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

见题面 Input 部分

见题面 Output 部分