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 proconio::input;
use proconio::marker::*;
use std::cmp::*;

fn main() {
    input! {
        n: u128
    }

    if n % 2 == 1{
        println!("0");
        exit();
    }

    let ten: f64 = 10.0;
    let max_times = (ten.log(5.0) * 18.0).ceil() as u32;
    let mut res = 0u128;
    for i in 1..max_times{
        res += n / (2 * 5u128.pow(i));
    }
    println!("{}",res);
}

fn exit(){
    std::process::exit(0);
}