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


val () =
    let
        val a = readInt ()
        val b = readInt ()

        fun findAns i =
            if b < i then 0
            else if (a + b + i) mod 3 = 0 then 1 + findAns (i + 1)
            else findAns (i + 1)
       
        val ans = findAns a
    in
        print (Int.toString ans ^ "\n")
    end