Problem:
Score : 500 points

Problem Statement

For an integer n not less than 0, let us define f(n) as follows:

f(n) = 1 (if n < 2)

f(n) = n f(n-2) (if n \geq 2)

Given is an integer N. Find the number of trailing zeros in the decimal notation of f(N).

Constraints

0 \leq N \leq 10^{18}

Input

Input is given from Standard Input in the following format:

N

Output

Print the number of trailing zeros in the decimal notation of f(N).

Sample Input 1

12

Sample Output 1

1

f(12) = 12 × 10 × 8 × 6 × 4 × 2 = 46080, which has one trailing zero.

Sample Input 2

5

Sample Output 2

0

f(5) = 5 × 3 × 1 = 15, which has no trailing zeros.

Sample Input 3

1000000000000000000

Sample Output 3

124999999999999995

Solution in Kotlin (1.3.71) (judge verdict: Wrong Answer)

fun main(args : Array<String>) {
    val n = readLine()!!.toLong()

    if (n % 2L == 1L) {
        println(0)
        return
    }

    var cntSum = 0L
    var ans = 0L
    for (i in 18 downTo 1) {
        val tmp = n / Math.pow(10.0, i.toDouble()).toLong()
        if (tmp > 0) {
            ans += (tmp - cntSum) * i
            cntSum += tmp - cntSum
        }
    }

    println(ans)
}