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 OCaml (4.02.3) (judge verdict: Accepted)

let () =
    let main () =
        let n = int_of_string (read_line ()) in
        let zeros = if n mod 2 = 1 then 0 else (
            let rec loop i t acc =
                if i > n then acc else
                    loop (i * 5) (t + 0) (acc + t * (n / i))
            in
            loop 10 1 0
        )
        in
        Printf.printf "%d\n" zeros
    in
    main ()