#include <fstream>
using namespace std;

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

int main ()
{
    int cnt,cnt2=-1,p=5;
    fin >> cnt;
    if (cnt<0)
        fout << -1;
    if (cnt==0)
        fout << 1;
    else
    {
        while (p<=cnt)
        {
            cnt2+=cnt/p;
            p*=5;
        }
        if (cnt2==-1)
            cnt2=0;
        fout << cnt*5-cnt2*5;
    }

    fin.close();
    fout.close();
    return 0;
}