#include <iostream>
#include <fstream>
using namespace std;
ifstream f("fact.in");
ofstream g("fact.out");
int cinci(int n)
{
    int nr=0;
    while(!(n%5))
    {
        nr++;
        n/=5;
    }
    return nr-1;
}
int main()
{
    int p,nr=0;
    f>>p;
    int n;
    n=p*5;
    for(int i=25;i<n;i+=25)
    {
        n=n-cinci(i)*5;
    }
    g<<n;
    return 0;
}