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 Rust (1.15.1) (judge verdict: Wrong Answer)

macro_rules! scan {
    ( $( $x:ident ),*; $t:ty ) => {
        let mut buf = String::new();
        let mut itr = {
            while buf.trim().len() == 0 {
                buf = String::new();
                std::io::stdin().read_line(&mut buf).unwrap();
            }
            buf.split_whitespace().map(|n| n.parse::<$t>().unwrap())
        };
        $(
            let $x = itr.next().unwrap();
        )*
    };
}

fn main() {
    scan!(n; u64);
    if n % 2 == 1 {
        println!("0");
        return;
    }

    let mut m:u64 = 0;
    let mut n = n;
    while n >= 5 {
        let t = n / 5;
        m += t;
        n = t;
    }

    println!("{}", m / 2);
}