let R () = stdin.ReadLine()

let N, M = 
    let t = R().Split() |> Array.map int
    in t.[0], t.[1]
let S = R().ToCharArray()
let T = R().ToCharArray()

let toMap = Array.countBy(fun x -> x) >> Map.ofArray
let getValue (m:Map<char,int>) c =
    match m.TryFind c with
    | Some(x) -> x
    | None    -> 0

let sMap = S |> toMap
let tMap = T |> toMap
let aCnt =
    let s = (sMap, 'A') ||> getValue
    let t = (tMap, 'A') ||> getValue
    min s t
let bCnt =
    let s = (sMap, 'B') ||> getValue
    let t = (tMap, 'B') ||> getValue
    min s t
let ans = aCnt + bCnt

ans |> printfn "%i"