#coding: utf-8
##yuki_63
import sys


s=map(int,raw_input().split())
l=s[0]
k=s[1]

if l<k*2:
 res=0
elif l==1 and k==1:
 res=0
elif l%2==1 and k==1:
 res=l/2
elif (l/2)%k==0:
 res=l/2-k
else:
 res=(l/2)/k*k
print res