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)

# coding: utf-8
N = gets.to_i

# ５の倍数で大きく割っていく
# 対象の数より大きくなるまでloop

if N.odd?
  puts 0
  exit 0
end

# 5の倍数を格納
m5 = 0
ans = 0
idx = 1

m5 = 5 ** idx

while N >= m5
  ans += (N / m5).to_i
  idx += 1
  m5 = 5 ** idx
#  p "#{m5}/#{ans}"
end

if ans == 0
  puts 0
else
  if ans < 10
    puts ans / 2
  else
    str = ans.to_s
    last = str[-1]
    ans2 = (ans / 10) / 2
    puts (ans2 * 10) + last.to_i
  end
end