
open Scanf
open Printf

let flip f x y = f y x

let rec range a b = if a >= b then [] else a :: range (a+1) b

let read_int () = scanf " %d" (fun x -> x)
let rec read_ints n = if n = 0 then [] else let i = read_int () in i :: read_ints (n-1)

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 rec for_iter_ a b f = if a >= b then () else f a; for_iter_ (a+1) b f

let list_of_string str =
  let len = String.length str in
  let rec f i = if i >= len then [] else str.[i] :: f (i+1) in
  f 0

let () = read_line ()
  |> list_of_string
  |> List.filter ((==) '1')
  |> List.length
  |> printf "%d\n"
