A - 台形 / Trapezoids

Time Limit: 2 sec / Memory Limit: 256 MB

この台形の面積を求めてください。

制約

• 1 \leq a \leq 100
• 1 \leq b \leq 100
• 1 \leq h \leq 100
• 入力で与えられる値はすべて整数
• h は偶数

入力

a
b
h


入力例 1

3
4
2


出力例 1

7


入力例 2

4
4
4


出力例 2

16


この例で与えられるのは平行四辺形ですが、平行四辺形も台形です。

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.