open System
open System.Collections.Generic

[<AutoOpen>]
module Cin =
    let read f = stdin.ReadLine() |> f
    let reada f = stdin.ReadLine().Split() |> Array.map f
    let readChars() = read string |> Seq.toArray
    let readInts() = readChars() |> Array.map (fun x -> Convert.ToInt32(x.ToString()))

[<AutoOpen>]
module Cout =
    let writer = new IO.StreamWriter(new IO.BufferedStream(Console.OpenStandardOutput()))
    let print (s: string) = writer.Write s
    let println (s: string) = writer.WriteLine s
    let inline puts (s: ^a) = string s |> println

let main() =
    let n = read int64
    match n with
    | _ when n < 2L -> 0L
    | _ when n % 2L <> 0L -> 0L
    | _ ->
        let mutable five = 0L
        let mutable v = 10L
        while v <= n do
            five <- five + (n / v)
            v <- v * 5L
        five
    |> puts
    ()

main()
writer.Close()
