l, k = list(map(int, input().split()))
if 2*k == l:
    print("0")
else:
    if l % (2*k) == 0:
        answer = (int(l / (2 * k)) - 1) * k
        print("{}".format(answer))
    else:
        answer = int(l / (2 * k)) * k
        print("{}".format(answer))