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)

fn main() {
    proconio::input! {
        n: usize,
    }
    let mut count2 = 0usize;
    let mut count5 = 0usize;
    if n % 2 != 0 {
        println!("0");
        return;
    }
    for d in 1.. {
        let div = 2usize.checked_pow(d).unwrap();
        let c = n / div;
        if c == 0 {
            break;
        } else {
            count2 += c;
        }
    }
    for d in 1.. {
        let div = 5usize.checked_pow(d).unwrap() * 2;
        let c = n / div;
        if c == 0 {
            break;
        } else {
            count5 += c;
        }
    }
    let ans = std::cmp::min(count2, count5);
    println!("{}", ans);
}