#include <iostream>
#include <fstream>
#include <math.h>
using namespace std;
ifstream fin("factorial.in");
ofstream fout("factorial.out");
unsigned long long x;
long l1,l2,n,mijl,p,q,i,s;
int main()
{
    fin>>n;
    q=sqrt(n);
    l1=1;
    l2=q;
    while (l1<=l2)
    {
        mijl=(l1+l2)/2;
        x=1;
        for (i=1;i<=mijl;i++)
            x=x*5;
        if (x>n)
            l2=mijl-1;
        else
        {
            p=mijl;
            l1=mijl+1;
        }
    }
    x=1;
    for (i=1;i<=p;i++)
    {
        x=x*5;
        s=s+n/x;
    }
    fout<<s;

    return 0;
}