fun readInt () =
    valOf (TextIO.scanStream (Int.scan StringCvt.DEC) TextIO.stdIn)


val () =
    let
        val n = readInt ()
        val k = readInt ()

        val ans = if k = 0 orelse k > n then 0
                  else if n = 1 then 0
                  else if n mod 2 = 1 andalso n - k = k - 1 then n - 1
                  else n - 2
    in
        print (Int.toString ans ^ "\n")
    end