fun main(args:Array<String>) {
    println(f(readLine()!!.toLong()))
}

fun f(num:Long):Long {
    if(num%2!=0L) {
        return 0
    }
    var tmp = num/10
    var cur = 50L
    for(i in (0..num)) {
        if(num < cur) {
            break
        }
        tmp += ((num/cur))
        cur *= 5
    }
    return tmp
}