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 Perl (v5.18.2) (judge verdict: Wrong Answer)

my $N = <STDIN>;
$mod = $N % 2;
if( $mod == 1){
  print "0";
  exit;
}
$ans = 0;
$a = $N/2;
for($i=0;$i<10000;$i++){ 
  $a = int($a / 5);
  $ans = $ans + $a;
  if($a ==0){last;}
}
print "$ans\n";