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;

fn main() {
    let n = parse_line::<i64>();
    println!("{}", double_fact_digit(n));
}

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

fn double_fact_digit(n: i64) -> i64 {
    if n % 2 != 0 {
        return 0;
    }
    let mut i = 1;
    let mut result: i64 = 0;
    loop {
        let law = 5_i64.pow(i) * 2;
        let adder = n / law;
        if adder == 0 {
            break;
        } else {
            result += adder;
        }
        i += 1;
    }
    result
}