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.chomp.to_i
# a = gets.chomp.split.map(&:to_i)

if n.odd?
  puts 0
  exit
end

def g1(n, p)
  return 0 if n == 0
  n/p + g1(n/p,p)
end

def g2(n, p)
  res = g1(n/2,p)
  res += n/2 if p == 2
  res
end

ans = g2(n,5)
puts ans