Scanf.scanf "%s" (fun s ->
    let n = String.length s in
    let rec loop i acc =
        if i > n - 7 then (if i <> n then acc ^ String.sub s i (n - i) else acc) else
        if String.sub s i 7 = "treeone" then (
            loop (i + 7) (acc ^ "forest")
        ) else (
            loop (i + 1) (Printf.sprintf "%s%c" acc s.[i])
        )
    in
    loop 0 "" |> print_endline
)