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-----
3
4
2

-----Sample Output-----
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.

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