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: Wrong Answer)

import sys
sys.setrecursionlimit(10**9)
N = int(input())
ans = 0
j = 0
tmp = 10
a = 0
while tmp <= N:
    tmp *= 5
    j += 1
j -= 1
tmp //= 5
b = 0
while j >= 0:
    b = N//tmp
#    ans +=  b
#    a += 5*b
    for i in range(j+1):
        ans += b * 5**i
    N -= b*tmp
    tmp //= 5
    j -= 1
#    print("N:{},ans:{},a:{},b:{},tmp:{},j:{}".format(N,ans,a,b,tmp,j))
print(int(ans))