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 Ruby (2.3.3) (judge verdict: Accepted)

n = gets.to_i

sum = 0

if n % 2 == 0 then
	len = n.to_s().size
	
	(0..1000000).each{ |i|
		base = (5 ** i) * 10
		
#		puts (n / base) * (2 ** i - 1) / (2 ** i)
		
		if n >= base then
			sum += n / base
		else
			break
		end
	}

	puts sum
else
	puts 0
end