Scanf.scanf "%d %d %s %s" (fun n m s t ->

    let count n s =
        let rec loop i a b =
            if i = n then a, b else
                if s.[i] = 'A' then loop (i + 1) (a + 1) b
                               else loop (i + 1) a (b + 1)
        in
        loop 0 0 0
    in

    let sa, sb = count n s in
    let ta, tb = count m t in

    Printf.printf "%d\n" @@ min sa ta + min sb tb
)