let n, a, b, c = Scanf.scanf " %d %s %s %s" @@ fun a b c d -> a, b, c, d
let ans = ref 0
let f a b c = if a = b && b = c then 0 else if a <> b && b <> c && c <> a then 2 else 1
let _ = String.iteri (fun i x -> ans := !ans + f x b.[i] c.[i]) a; Printf.printf "%d\n" !ans