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: Accepted)

use std::io;
use std::str::FromStr;

fn input<T:FromStr>() -> T {
    let mut line: String = String::new();
    io::stdin().read_line(&mut line);
    line.trim().parse().ok().unwrap()
}

fn calc(n: u64) -> u64 {
    let mut digit = 27u32;
    let mut buf = 0;
    let mut sum = 0;
    while digit > 0 {
        let m = n / 5u64.pow(digit) / 2;
        // println!("{}, {}, {}", digit, m, buf);
        // println!("{}", (m - buf) * digit as u64);
        sum += (m - buf) * digit as u64;
        buf = m;
        digit -= 1;
    }
    sum
}

fn main() {
    let n: u64 = input();
    if n % 2 == 1 {
        println!("0");
    } else {
        println!("{}", calc(n));
    }
}