R, C = map(int, input().split())

if R == C:
    # Square case
    if R % 2 == 1:
        s = 1
    else:
        s = 0
    eq = s + (R * R - s) // 4
else:
    # Non-square case
    a = 1 if (R % 2 == 1 and C % 2 == 1) else 0
    eq = (R * C + a) // 2

print(eq - 1)