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)

def fac0(a):
    r = 0
    while a > 0:
        r += a//5
        a = a//5
        #print("r=",r)
        #print("a=",a)
    return(r)

N = int(input())
#print("a=",N//2)
#for i in range(999999999999999900,1000000000000000001):
#    N = i
#    print("i=",i)
    
if N % 2 == 0:
    print(fac0(N//2))
else:
    print(0)