
def main():
    line = input()
    array = line.split(' ')
    m = int(array[0])
    n = int(array[1])
    print(int((m*n)/2))


if __name__ == "__main__":
    main()

a77c5f0e43836f98c4d7116cef5f3ea27f44366b
<mask token>

def main():
    line = input()
    array = line.split(' ')
    m = int(array[0])
    n = int(array[1])
    print(int(m * n / 2))


<mask token>

def main():
    line = input()
    array = line.split(' ')
    m = int(array[0])
    n = int(array[1])
    print(int(m * n / 2))


if __name__ == '__main__':
    main()


def main():
    line = input()
    array = line.split(' ')
    m = int(array[0])
    n = int(array[1])
    print(int((m*n)/2))


if __name__ == "__main__":
    main()
