text: #!/bin/python3
  
  import sys
  
  
  n,m = input().strip().split(' ')
  n,m = [int(n),int(m)]
  
  no_of_boxes = n * m
  no_of_quad_neighbours = no_of_boxes // 4
  remainder = no_of_boxes % 4
  if remainder:
      no_of_duo_boxes = remainder // 2
      loner = remainder % 2
      if loner:
          additional_bases = no_of_duo_boxes + 1
      else:
          additional_bases = no_of_duo_boxes
  else:
      additional_bases = 0
      
  no_of_bases = no_of_quad_neighbours + additional_bases
  
  print(no_of_bases)
  