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)

def fn(n)
  res = 1
  n.step(1, -2) do |i|
    break if n < 2
    res *= i
  end
  res
end

def count_r(n)
  rev = n.to_s.reverse
  res = 0
  rev.chars.each { |i| break if i != '0'; res += 1 }
  res
end

n = gets.to_i
if n.odd?
  puts "0"
  exit
end

# prepared_res = count_r(fn(n))
res = 0
res += n / 10
n /= 10
while n > 5
  res += n / 5
  n /= 5
end
# res += n / 250
puts res
# puts prepared_res
# puts count_r  fn(10)
# puts count_r  fn(20)
# puts count_r  fn(30)
# puts count_r  fn(40)
# puts count_r  fn(50)
# puts count_r  fn(80)
# puts count_r  fn(96)
# puts count_r  fn(100)
# puts count_r  fn(110)
# puts count_r  fn(140)
# puts count_r  fn(150)