Sovet

total = int(input())
parents = int(input())

if parents / total >= 1 / 3:
    print(0)
else:
    l = 0
    r = total - parents

    while r - l > 1:
        mid = l + (r - l) // 2
        if (parents + mid) / (total + mid) < 1 / 3:
            l = mid
        else:
            r = mid

    print(r)