(R, C) = map(int, input().split())
if R == C:
  if R%2 == 0:
    print(int(R*C/4)-1)
  else:
    print(int((R+1)/2)*int(C/2))
elif R%2 == 0 or C%2 == 0:
  print(int(C*R/2)-1)
else:
  print(int(C*(R-1)/2)+int(C/2))