h,w=map(int,input().split())
if (h%2,w%2)==(0,0):
  print((h*w//2 if h!=w else h*w//4)-1)
if (h%2,w%2)==(0,1):
  print((h*w//2)-1)
if (h%2,w%2)==(1,0):
  print((h*w//2)-1)
if (h%2,w%2)==(1,1):
  print(((h*w-1)//2+1 if h!=w else (h*w-1)//4+1)-1)