#include<fstream.h>
main()
{long p,n=0,cont=1,c;
 ifstream i("fact.in");
 ofstream o("fact.out");
 i>>p;
 while(p--)
  {n=n+5;
   c=cont;
   while(cont%5==0)
    {p--;cont/=5;}
   cont=c;
   cont++;
  }
 o<<n<<'\n';

 i.close();
 o.close();
 return 0;
}