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 Python (3.8.2) (judge verdict: Runtime Error)

# 148e

from sys import stdin
import re
import numpy as np
import math

#stdin = open("test/abc148_e_sample-3.in")
def get_rank(N):
    rank = int(np.log(N)/np.log(5))
    #print(rank)
    
    num = 0
    
    for i in range(1, rank+1):
        num_i = N//(2*5**i)
        #print(num_i)
        num += num_i
    
    return num

N = int(stdin.readline().strip())

if N%2==1 or 0:
    print(0)
else:
    print(get_rank(N))