from decimal import *

r, c = map(int, input().split())

if r % 2 == 0 and  c % 2 == 0:
    r, c = c, r
    
if r == c:
    print(Decimal((Decimal(r) * Decimal(c) + 3) / 4) - 1)
else:
    print(Decimal((Decimal(r) * Decimal(c) + 1) / 2) - 1)