l, k = map(int, input().split())

if l <= k:
    print(0)
else:
    yu = (l // (2 * k) - 1) * k
    l -= yu * 2

    while l > 0:
        if l > 2 * k:
            yu += k
            l -= 2 * k
        elif l <= 2 * k:
            l -= l

    print(yu)