# coding: utf-8

l, k = map(int,input().split())
#l = int(input())
#N = int(input())
#W = list(map(int,input().split()))

if l % (2 * k) == 0:
    cnt = l // (2 * k) - 1
else:
    cnt = l // (2 * k)

print(cnt * k)