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 PHP7 (7.0.15) (judge verdict: Wrong Answer)

<?php

function calc($n, $acc)
{
    if ($n < 2) {
        return $acc;
    }

    return calc($n - 2, $acc * $n);
}

$h = STDIN;
$n = trim(fgets($h));

$tail = (int)$n[strlen($n) -1 ];

if ($tail % 2 !== 0) {
    echo 0;
}else{
    echo -1;
}
die;

$res = (string)calc($n, 1);
var_dump($res);die;
$ans = 0;
for ($i = strlen($res); $i >= 0; $i--) {
    if ($res[$i - 1] === '0') {
        ++$ans;
    } else {
        break;
    }
}
echo $ans;