open Printf open Scanf
open Array

let print_array f a = Array.iter (fun v -> printf "%s " @@ f v) a; print_newline ()

let () = scanf " %d %s" @@ fun n s ->
  let a = init n (fun i -> s.[i]) in
  let res = ref 0 in
  let rec lp i black =
    let rec lp2 i =
      if i<n && a.(i) = '.' then lp2 (i+1)
      else i
    in
    if i=n then ()
    else if a.(i-1) = '#' && black > 0
      && a.(i) = '.' then (
      let j = lp2 @@ i+1 in
      let white = j - i in
      if black >= white then (
        res := !res + white;
        lp j (black + white);
      ) else (
        res := !res + black;
        lp j 0;
      );
    ) else lp (i+1) (black + if a.(i) = '#' then 1 else 0)
  in
  lp 1 (if a.(0) = '#' then 1 else 0);
  print_int !res;