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: Runtime Error)

S = {}

A = input()
B = input()
C = input()

S['a'] = A
S['b'] = B
S['c'] = C

player = 'a'
for i in range(len(A) + len(B) + len(C)):
    # pass
    # print(S[player][0])
    S[player] = S[player][1:]
    
    if len(S[player]) == 0:
        break
    
    player = S[player][0]
    

# print(A)
# print(B)
# print(C)

print(player.upper())