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: Wrong Answer)

n = gets.chomp.to_i

k = n.to_s.size

if n.odd?
  p 0
  exit
end

ans = 0
1.upto(k-1) do |i|
  ans += n / (10 ** i)
end

1.upto(100) do |i|
  f = (5 ** i) * 10
  if f > n
    break
  end
  m = n / f
  ans += (m / 2.0).ceil
  1.upto(100) do |p|
    g = f * (10 ** p)
    break if g > n
    h = n / g
    ans += (h / 2.0).ceil
  end
end
p ans