let safeCheesesNum n k =
    let (|CorrectMsg|ErrorMsg|) (n,k) =
        match 0 < k && k <= n with | true -> CorrectMsg | _ -> ErrorMsg
    match n,k with
    | ErrorMsg   -> 0
    | CorrectMsg ->
        let (|Odd|Eve|) n = match n%2 with | 0 -> Eve | _ -> Odd 
        match n with
        | Eve -> n-2
        | Odd -> if (n/2)+1 = k then n-1 else n-2

let N, K = let t = stdin.ReadLine().Split() |> Array.map int
           in t.[0], t.[1]

safeCheesesNum N K |> printfn "%i"