R, C = map(int, input().split())
if R == C:
    if R % 2:
        print((R * C - 1)//4)
    else:
        print(R * C // 4 - 1)
else:
    if R * C % 2:
        print((R * C - 1)//2)
    else:
        print(R * C // 2 - 1)