#include<fstream>
using namespace std;
int main()
{
    int P,N,i,s,k;
    ifstream f("fact.in");
    ofstream g("fact.out");
    f>>P;
    N=0;
    for(i=1;i<=P;i++)
    {
        N=N+5;
        if(N%25==0&&N>=25) s=N/5;
        k=0;
        while(s>=5)
        {
            s=s/5;
            k=k+5;
        }
      N=N-k;


    }
  g<<N;


}