let string_1_list_of_string x =
  String.to_seq x
  |> List.of_seq
  |> List.map (String.make 1);;

let hikaku = string_1_list_of_string "abcdefghijklmnopqrstuvwxyz";;

let rec check lst lst2 =
  let c1 = List.hd lst in
  let c2 = List.hd lst2 in
  if (c1 <> c2) then (c1 ^ "to" ^ c2)
  else check (List.tl lst) (List.tl lst2);;

let () =
  let test = read_line () |> string_1_list_of_string in
  check hikaku test |> print_string; print_newline ();;