s = input().split()
W = int(s[0])
H = int(s[1])

if W % 2 == 0:
    out = (H * W) / 2
else:
    if W % 2 == 0:
        k = H / 2
    else:
        k = H / 2+1
    out = int((H * (W - 1)) / 2) + int(k)

print(int(out))
