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)

lines = STDIN.read.strip.lines.map(&:strip)

n = lines.first.to_i

def f(num)
    return 1 if num < 2
    num * f(num-1)
end

if n % 2 == 1
    puts 0
    exit(0)
end

curr = n
cnt = 0
while curr % 5 != 0
    curr = curr - 1
end


while curr > 9
    # puts curr
    time = 0
    _curr = curr
    while true
        if _curr % 5 == 0
            # p _curr
            cnt += 1
            _curr = _curr / 5
        else
            break
        end
    end
    curr = curr - 10
end

puts cnt