#include<fstream.h>
main ()
{long n,p,t=25,c=1;
 ifstream i("fact.in");
 ofstream o("fact.out");
 i>>p;n=p*5;
 for(t=25;t<n;t=t*5)
  {n=n-5*c;c++;}
 if(n==0)
  n++;
 o<<n<<'\n';
 i.close();
 o.close();
 return 0;
}