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

if r == c:
    if r%2:
        print((r+1)//2*(c+1)//2-1-r//2)
    else:
        print(r//2*c//2-1)

else:
    if r%2 == 0 and c %2 == 0:
        print(r*c//2-1)
    elif r%2 and c%2:
        print(max((r+1)//2*c-c//2-1,(c+1)//2*r-r//2-1))

    else:
        print(r*c//2-1)