问题 C: idol!!

Akuya has finally obtained Shinki Hikari's iPhone. Now, he needs to crack the password to gather evidence!

Blindly attempting to crack the password won't work, so Akuya found a note inside Shinki Hikari's phone case. On it was a puzzle! He believes the solution to this puzzle is the phone's password. The puzzle is as follows:

Given a positive integer $n$, the double factorial of $n$ is the product of all positive integers with the same odd/even parity as $n$ and not exceeding $n$. It is denoted as $n!!$, for example, $5!!=1\times 3\times 5,6!!=2\times 4\times 6$.

Find the number of trailing zeros in the decimal representation of the product $1!!\times 2!!\times 3!!\times \cdots \times n!!$.

Akuya is just one step away from achieving his revenge. Can you help him?

Input one line containing a positive integer n(1≤n≤1018)n (1 \leq n \leq 10^{18})n(1≤n≤1018).

Only one line, containing a integer, represents the answer.

11

5

$1!!\times 2!!\times 3!!\times \cdots \times 11!!$ = $52563198423859200000$