let solve n k =
  if k <= 0 || k > n then 0
  else if n mod 2 = 1 && k = n / 2 + 1 then n - 1
  else n - 2

let () =
  let n, k = read_line ()
             |> Str.split (Str.regexp_string " ")
             |> List.map int_of_string
             |> fun arr -> (List.nth arr 0, List.nth arr 1)
  in
  solve n k
  |> string_of_int
  |> print_endline