R,C = map(int,input().split())
if R>=C:
    R,C = C,R
if R<C:
    if R%2==0:
        ans = (R//2)*C
    else:
        ans = (R//2)*C+(C+1)//2
else:
    if R%2==0:
        ans = (R//2)**2
    else:
        ans = (R//2)*(R//2+1)+1
print(ans-1)