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)

from itertools import combinations, product


def gcd(a, b):
    if a < b:
        a, b = b, a
    while b != 0:
        a, b = b, a % b
    return a

def lcm(a, b):
    return a * b // gcd(a, b)


def get(fmt):
    ps = input().split()
    r = []
    for p, t in zip(ps, fmt):
        if t == "i":
            r.append(int(p))
        if t == "f":
            r.append(float(p))
        if t == "s":
            r.append(p)
    if len(r) == 1:
        r = r[0]
    return r

def put(*args, **kwargs):
    print(*args, **kwargs)


def rep(n, f, *args, **kwargs):
    return [f(*args, **kwargs) for _ in range(n)]
    
def rep_im(n, v):
    return rep(n, lambda: v)
    
YES_NO = ["NO", "YES"]


a, b, h = rep(3, get, "i")

put((a + b) * h // 2)