def main():
    r, c = map(int, input().split())

    if r != c:
        if r % 2 == 1 and c % 2 == 1:
            ans = (r * c - 1) // 2
        else:
            ans = r * c // 2 - 1
    else:
        if r % 2 == 0:
            ans = (r * c) // 4 - 1
        else:
            ans = (r * c - 1) // 4

    print(ans)


if __name__ == "__main__":
    main()