import math

r, c = map(int, input().split())

ans = 0

if r != c:
    if r % 2 == 0 or c % 2 == 0:
        ans = r * c // 2 - 1
    elif r % 2 != 0 and c % 2 != 0:
        ans = math.floor(r * c / 2)
else:
    if r % 2 == 0 and c % 2 == 0:
        ans = r * c // 4 - 1
    elif r % 2 != 0 and c % 2 != 0:
        ans = math.floor(r * c / 4)

print(ans)