a = int(eval(input()))

b = int(eval(input()))

h = int(eval(input()))

print(((a+b) * h // 2))
def main():

    a = int(eval(input()))

    b = int(eval(input()))

    h = int(eval(input()))

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





if __name__ == '__main__':

    main()

a = int(eval(input()))
b = int(eval(input()))
h = int(eval(input()))
print(((a + b) * h // 2))

def main():
    a = int(eval(input()))
    b = int(eval(input()))
    h = int(eval(input()))
    print(((a + b) * h // 2))


if __name__ == "__main__":
    main()

-a = int(eval(input())) -b = int(eval(input())) -h = int(eval(input())) -print(((a + b) * h // 2)) +def main(): +    a = int(eval(input())) +    b = int(eval(input())) +    h = int(eval(input())) +    print(((a + b) * h // 2)) + + +if __name__ == "__main__": +    main()