r, c = map(int, input().split())
if r != c:
    if r & 1 and c & 1:
        print(r * c // 2)
    else:
        print(r * c // 2 - 1)
else:
    if r & 1 and c & 1:
        print((r // 2) ** 2 + (r + 1) // 2 - 1)
    else:
        print(r * c // 4 - 1)