
fun main(args: Array<String>) {
    val n = readLine()!!.toLong()
    if (n % 2L == 1L) {
        println(0)
        return
    }

    println(xx(n))
}

private fun xx(x: Long): Long {
    var e = 0L
    var div = 10L
    while (div <= x) {
        e += (x / div)
        div *= 5
    }
    return e
}
