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.