let a, b = stdin.ReadLine().Split() |> (fun x -> int x.[0], int x.[1])
let rec hoge x =
    if x > b then 0
    elif (a + b + x) % 3 = 0 then 1 + (hoge (x + 1))
    else hoge (x + 1)
printfn "%d" (hoge a)