n, m = map(int, input().split())
total = (n * m) // 4
if n == 1 or m == 1:
    print(total)
else:
    a = n // 2
    b = m // 2
    odd_rows = n % 2
    odd_cols = m % 2
    max_os = a * b
    add = (odd_rows * b + odd_cols * a) // 2
    max_os += add
    i = max(0, total - max_os)
    print(i)