def main():

    a = int(eval(input()))

    b = int(eval(input()))

    h = int(eval(input()))

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





if __name__ == '__main__':

    main()

from collections import defaultdict





def main():

    a, b, h = int(eval(input())), int(eval(input())), int(eval(input()))

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





if __name__ == '__main__':

    main()

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


if __name__ == "__main__":
    main()

from collections import defaultdict


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


if __name__ == "__main__":
    main()

+from collections import defaultdict + + -    a = int(eval(input())) -    b = int(eval(input())) -    h = int(eval(input())) +    a, b, h = int(eval(input())), int(eval(input())), int(eval(input()))