101484: [AtCoder]ABC148 E - Double Factorial

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