let () =
  let n, k = Scanf.scanf "%d %d " (fun n k -> n, k) in
  if k <= 0 || k > n || n = 1 then print_endline "0"
  else if n mod 2 = 0 then Printf.printf "%d\n" (n - 2)
  else Printf.printf "%d\n" (n - if (n + 1) / 2 = k then 1 else 2)