fun main(args: Array<String>) {
    val N = readInputLine().toLong()

    if (N % 2 == 1L) {
        println(0)
    } else {
        var ans = 0L
        var div = 10L
        while (div <= N) {
            ans += N / div
            div *= 5L
        }
        println(ans)
    }
}

fun readInputLine(): String {
    return readLine()!!
}

