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 (3.4.3) (judge verdict: Accepted)

N=int(input())
if N%2==1:
  print(0)
  exit()
def func1(n):
  if n==0:
    return 0
  if n==1:
    return 1
  return 5*func1(n-1)+1
def func(x):
  if x==0:
    return 0
  for i in range(26):
    if x<5**(i+1):
      a=i
      b=x//(5**a)#商
      c=x%(5**a)#あまり
      break
  return b*func1(a)+func(c)
print(func(N//2))