l, k = map(int, input().strip().split())
ly = 0
lh = 0
while l>0:
    if l > 2*k:
        l = l - 2*k
        ly += k
        lh += k
    elif l > k:
        l = l -k
        lh += k
    else:
        l = 0
        lh += l
print(ly)