#include <fstream>

using namespace std;

ifstream fin("fact.in");
ofstream fout("fact.out");

int n;

int fact(int x)
{
    int r=0;
    while (x>=5) {
        r++;
        x/=5;
    }
    return r;
}

int main()
{
    int s=0;
    fin>>n;
    for (int i=5;i<=n;i+=5) s+=fact(i);
    fout<<s;
}