R,C = gets.split.map(&:to_i).sort

if R.odd? && C.odd?
    p (R)*(C)/4+1-1 if R == C
    p (R)*(C)/2+1-1 if R != C
elsif R.even? && C.odd?
    p C*(R/2)-1
elsif R.odd? && C.even?
    p (C)*(R/2)+C/2-1
elsif R.even? && C.even?
    p R*C/4-1 if R == C
    p R*C/2-1 if R != C
end