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Score : $100$ points

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.

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.

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.