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 Go (1.6) (judge verdict: Wrong Answer)

package main

import (
	"fmt"
)

func main(){
	var number int64
	fmt.Scanf("%d", &number)

	fmt.Printf("%d\n", answer(number))
}

func answer(n int64) int64 {
	ret := int64(0)
	for i := 1; i < 2000; i++ {
		t := int64(1)
		for j := 1; j <= i; j++ {
			t = t * 5
		}
		if n < (t * 2) {
			break
		}
		ret += n / (2 * t)
	}
	return ret
}