l,k = map(int,input().split())
#print(((l//2)//k)*k if l%k!=0 else l//2-k)
if k == 1:
    print(l//2 if l%2!=0 else l//2-1)
elif l/k == 2:
    print(0)
else:
    print((l//2)//k*k)