n, m = map(int, input().split())

total = (n * m) // 4

if n == 1 or m == 1:
    max_os = 0
else:
    if n % 2 == 0 and m % 2 == 0:
        max_os = total
    elif n % 2 == 1 and m % 2 == 1:
        max_os = (n * m - 1) // 4
    else:
        max_os = (n // 2) * (m // 2)

min_i = max(0, total - max_os)
print(min_i)