stdin.ReadLine().ToCharArray()
|> Array.indexed
|> Array.fold(fun (mem,counter,maxLen) (idx,c) ->
    let counter = match c with |'A' -> counter + 1 | _  -> counter - 1        
    match Map.containsKey counter mem with
    | true ->  (mem, counter, (max maxLen (idx - mem.[counter])))
    | _    ->  ((mem.Add(counter,idx)), counter, maxLen)) (Map.ofList[0,-1],0,0)    
|> fun (_,_,maxLen) -> printfn "%i" maxLen