R, C = map(int, input().split())
if R == C and R & 1:
    K = (1 + R // 2) * (C // 2)
elif R == C:
    K = (R // 2) * (C // 2) - 1
else:
    if (R * C) & 1:
        K = 2 * (R // 2) * (C // 2) + (C // 2) + (R // 2)
    else:
        K = (R * C) // 2 - 1
print(K)