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 Java8 (OpenJDK 1.8.0) (judge verdict: Wrong Answer)

import java.util.*;

public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		String Ns = sc.next();
		sc.close();
		long N = Long.parseLong(Ns);
		long cal = 1;
		long now = N;
		for (long n = 0; n < N ; n++){
			if (now == 0 || now == 1) {
				break;
			} else {
				cal = cal * now;
				now = now - 2;
			}
		}
		long zero = 0;
		long zerocal = cal;
		for (long i = 0;; i++){
			if (zerocal % 10 == 0){
				zerocal = zerocal / 10;
				zero++;
			} else {
				break;
			}
		}
		System.out.println(zero);
	}}