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 Python (3.4.3) (judge verdict: Accepted)

#上底の長さ
a = input()
#下底の長さ
b = input()
#高さ
h = input()
#答え
ans = 0
#文字列を数値化しつつ
#(上底+下低)*高さ/2で台形の面積を求める
ans = (int(a) + int(b)) * int(h) / 2
#答えを出力
#小数点以下が表示されるのでround()で非表示にしている
print(round(ans))