open System

let countAB str =
    str
    |> Seq.toList
    |> List.partition (fun c -> c = 'A')
    |> fun (aLst, bLst) -> (List.length aLst, List.length bLst)

let solve s t =
    let a1, b1 = countAB s
    let a2, b2 = countAB t
    Math.Min(a1, a2) + Math.Min(b1, b2)

let () =
    Console.ReadLine() |> ignore
    let s = Console.ReadLine()
    let t = Console.ReadLine()
    solve s t
    |> printfn "%d"