let S = stdin.ReadLine().ToCharArray()

let ans = 
    let mem = System.Collections.Generic.Dictionary<int,int>()
    mem.Add(0,-1)
    S 
    |> Array.indexed
    |> Array.fold(fun (counter,maxLen) (idx,c) ->
        let counter = match c with |'A' -> counter + 1 | _  -> counter - 1
        match mem.TryGetValue counter with
        | true, lIdx -> (counter, max maxLen (idx - lIdx))
        | _          -> mem.Add(counter,idx); (counter, maxLen)) (0,0)
    |> fun (counter,maxLen) -> maxLen
    
ans |> printfn "%i"