You are given a trapezoid. The lengths of its upper base, lower base, and height are a, b, and h, respectively.

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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

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