Problem:
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.

Solution in Ruby (2.3.3) (judge verdict: Wrong Answer)

X = {"a" => gets.chomp, "b" => gets.chomp, "c" => gets.chomp}

X["turn"] = "a"

while (X["a"].length)*(X["b"].length)*(X["c"].length) > 0
	X[X["turn"]].slice!(0)
	X["turn"] = X[X["turn"]][0] || X["turn"]
end


puts X["turn"].upcase