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: Runtime Error)

import math
def f(n):
    if n<2:
        return 1
    else:
        return n * f(n-2)
N=int(input())

# 5,2で10, 10で10
# nについて5のカウント数をa, 2のカウント数をbとすると，min(a,b)がカウント数となる
if not N%2==0:
    print(0)
else:
    count = 0
    max_n = int(math.log(N,5))
    for i in range(1,max_n+1):
        count+=int(int(N/(5**i))/2)
    
    print(count)