w = list(map(int,input().split()))

if w[0] <= 2 * w[1]:
  print(0)
elif w[0]%(2*w[1])==0:
  print(int(((w[0]//(2*w[1]))-1)*w[1]))
else:
  print(int((w[0]//(2*w[1]))*w[1]))