fun main(args: Array<String>) {
    val n = readLine()?.toLong() ?: return
    if (n % 2L == 1L) {
        println("0")
    } else {
        var d = 5L
        var total = 0L
        while (d <= n) {
            total += (n / d) / 2
            d *= 5L
        }
        println(total.toString())
    }
}