let () =
    let main () =
        let n = int_of_string (read_line ()) in
        let zeros = if n mod 2 = 1 then 0 else (
            let rec loop i t acc =
                if i > n then acc else
                    loop (i * 5) (t + 0) (acc + t * (n / i))
            in
            loop 10 1 0
        )
        in
        Printf.printf "%d\n" zeros
    in
    main ()