Description

Enter the upper bottom, the lower bottom and the height of the trapezoid. Calculate the area of the trapezoid then output it.

Input Format

Three integers.Enter the upper bottom,the lower bottom and the height of the rectangle.

Output Format

An integer,which is the area of the rectangle.

2 6 3

12

Limit and hint

For $100\%$ testcases， $a,b,h\leq10^5$.