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"
	"bufio"
	"os"
	"strings"
	"log"
	"strconv"
)
 
var sc = bufio.NewScanner(os.Stdin)

func Rr() []string {
  sc.Scan()
  slice := strings.Split(sc.Text(), " ")
  return slice
}
 
func abc148e(N uint64){
  if N % 2 != 0{
    fmt.Println("0")
  }else{
    fmt.Println(" ")
  }
}
 
func main(){
	N := uint64(0)
	fmt.Scan(&N)
	abc148e(N)
}

//入力関係ライブラリ(Atcoder:yamanobori_oldさんのもの)
func make_liner(maxByte int) *bufio.Scanner {
  liner := bufio.NewScanner(os.Stdin)
  liner.Buffer(make([]uint8, 0, 8192), maxByte)
  return liner
}
func word_spliter() *bufio.Scanner {
  words := make_liner(8192) 
  words.Split(bufio.ScanWords)
  return words
}

func eGetLine(r *bufio.Scanner) string {
  if r.Scan() {
    return r.Text()
  }
  err := r.Err()
  if err != nil {
    log.Fatal(err)
  }
  // EOF
  return ""
}

func eGetInt(r *bufio.Scanner) int {
  line := eGetLine(r)
  return eAtoi(line)
}

func eAtoi(s string) int {
  n, err := strconv.Atoi(s)
  if err != nil {
    log.Fatal(err)
  }
  return n
}