R, C = [int(i) for i in input().split()]

if R == C:
    if R & 1 == 0:
        k = (R >> 1) * (C >> 1)
    else:
        k = (R >> 1) * (C >> 1) + (R >> 1) + 1
else:
    if R & 1 == 0:
        k = (R >> 1) * C
    elif C & 1 == 0:
        k = (C >> 1) * R
    else:
        k = (R >> 1) * C + (C >> 1) + 1

print(k-1)