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

    var result = 0L
    var current = 5L
    while (current < N) {
        result += (N / current) / 2
        current *= 5
    }
    println(result)
}