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[] argv){
    Scanner sc = new Scanner(System.in);

    long n = sc.nextLong();

    if (n < 2){
      System.out.println(0);
      return;
    }

    if (n % 2 != 0){
      System.out.println(0);
      return;
    }

    long ans = 0;
    for (int i = 0; i <= 30; i++) {
      long num = (long)Math.floor(n / 10 / Math.pow(5, i));
      ans += num;
    }

    System.out.println(ans);
    return;
  }
}