program main
  implicit none
  integer*8::N,K
  read *,N,K
  if(N.lt.K.or.0.ge.K) then
     print '("0")'
  else if(N+1.eq.2*K) then
     print '(i0)',N-1
  else
     print '(i0)',N-2
  end if
end program main