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 (1.42.0) (judge verdict: Accepted)

use std;
use std::collections::VecDeque;
fn read<T: std::str::FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().parse().ok().unwrap()
}

fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    read::<String>()
        .split_whitespace()
        .map(|e| e.parse().ok().unwrap())
        .collect()
}

fn read_vec2<T: std::str::FromStr>(n: u32) -> Vec<Vec<T>> {
    (0..n).map(|_| read_vec()).collect()
}

/*
ここまでテンプレ
*/
fn ord(a: (i32, i32), b: (i32, i32)) -> bool {
    if (a.0 < b.0) {
        true
    } else {
        false
    }
}
fn main() {
    let N = read::<i128>();
    if N % 2 == 1 {
        println!("0");
        return;
    }
    let mut ans: i128 = 0;
    let mut tmp: i128 = 10;
    loop {
        if (N / tmp) == 0 {
            break;
        }
        ans += N / tmp;
        tmp *= 5;
    }
    println!("{}", ans);
}