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
if n % 2 !=0
  puts 0
elsif n == 0
  puts 0
else
  n1 = n/2
  count_5 = 0
  divisor = 5
  while  n1 > divisor  do
    count_5 += n1/divisor
    divisor = 5*divisor
  end
  count_2 = 0
  divisor2 = 2
  while n1 > divisor2 do
    count_2 += n1/divisor
    divisor2 = 2*divisor
  end
   count_2 += n/2
  if count_5 > count_2
  puts count_5
  else
  puts count_2
  end
end