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 Fortran (gfortran v4.8.4) (judge verdict: Accepted)

program abc148e
    use,intrinsic :: iso_fortran_env
    implicit none
    integer(int64):: n
    integer(int64):: div,ans

    read*, n
    if (n < 10) then
        print'(i0)', 0
        stop
    end if

    if (mod(n,2_8) == 1) then
        print'(i0)', 0 
        stop
    end if
    div=10
    ans=0
    do while (div <= n)
        ans = ans + n/div
        div=div*5
    end do
    print'(i0)', ans
end program abc148e