R, C = map(int, raw_input().split())
if R == C:
    print sum(max(i-1, 1) for i in xrange(2-R%2, R+1, 2))-1
else:
    if R > C:
        R, C = C, R
    print C*(R/2) + (R%2)*(C/2+C%2) -1