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 (2.4.0) (judge verdict: Wrong Answer)

import math

def count(L):
  K=0
  for l in range(L):
    K+=10**(L-l-1)
  for l in range(L-1):
    K+=10**(L-l-2)
  return K

N=int(input())

if N%2==1:
  print(0)
else:
  R = 0
  while True:
    if N<10:
      break
    L=int(math.log10(N))
    M=int(N/10**L)
    R+=M*count(L)
    N = N-M*10**L
  print(R)