H, W = map(int, input().split())

N1 = min(H, W)
N2 = max(H, W)

if N1 % 2 == 0:
  count = (N1 * N2) // 2
else:
  count = (N1 * (N1 -1) + N1 + 1) // 2
  if (N2 - N1) % 2 == 0:
    count += (N1 * (N2 - N1)) // 2
  else:
    count += (N1 * (N2 - N1 - 1) + N1 - 1) // 2

print(count)