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: Accepted)

import java.util.Scanner;

public class Main {
	static long solve(long N){
		if(N % 2 == 1){
			return 0;
		}
		long m = 5;
		long S = 0;
		while(m < N){
			S += N / (m * 2);
			m = m * 5;
		}
		return S;
	}
	public static void main(String[] args) {
//		for(int N = 1 ; N <= 100 ; ++N){
//			System.out.println(N+" "+solve(N));
//		}
		Scanner sc = new Scanner(System.in);
		long N = sc.nextLong();
		System.out.println(solve(N));
	}
}