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 C (Clang 3.8.0) (judge verdict: Wrong Answer)

#include<stdio.h>
long long sum = 0;
void fa(long long n)
{
    if(n > 5)
    {
        sum += n / 5;
        return fa(n/5);
    }
    else return;
}
int main()
{
    long long  n;
    scanf("%lld",&n);
    if(n % 2) printf("0\n");
    else
    {
        fa( n / 2);
        printf("%lld",sum);
    }
    return 0;
}