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 PyPy3 (7.3.0) (judge verdict: Accepted)

def count_five(n):
    res = 0
    i = 1
    while n//pow(5,i) - n//pow(10,i)> 0:
        res += n//pow(5,i) - n//(2*pow(5,i))
        i += 1
    return res

def main(N):
    if N%2:
        return 0
    M = list(str(N))
    L = len(M)
    temp = 0
    temp2 = 0
    res = 0
    for i, n in enumerate(M[:-1], start=1):
        temp *= 10
        temp += int(n)
        res += (temp - temp2)*(L-i)
        res += count_five(temp)
        temp2 = temp
    return res

N = int(input())
print(main(N))