line = input()
data = line.split()
 
height = int(data[0])
wei = int(data[1])
 
if height %2 ==0:
    print(int(height/2)*wei)
else:
  if wei %2 ==0:
    print(int(height/2)* wei + int(wei/2))
  else:
    print(wei* int(height/2) + int(wei/2)+1)
