let list_of_str s = let rec f i = if i >= String.length s then [] else s.[i] :: f (i+1) in f 0
let rec for_iter a b f = if a > b then [] else let x = f a in x :: for_iter (a+1) b f
let () =
  let s = Scanf.scanf "%s" list_of_str in
  for_iter 97 (97+25) (fun i ->
    let c = char_of_int i in
    List.filter ((=) c) s |> List.length |> (fun x -> x mod 2))
  |> List.fold_left (+) 0
  |> (fun x -> Printf.printf "%d\n" (x / 2))
