#include<fstream>
using namespace std;

ifstream f("fact.in");
ofstream g("fact.out");

int main()
{
unsigned long p,i,total=0,cp,intr=0;
bool nu = true;
bool lf = false;

f>>p;
for(i=1; ( i <= 10000000000 ) && ( nu == true ); i++)
    {
      cp = i;
      intr=0;
      if(total > p)
         break;
      else if(total == p)
         {
          lf = true;
          break;
         }
      else while(cp % 5 == 0)
                {
                 cp/=5;
                 intr++;
                }
      total+=intr;
    }

if(lf == false)
   g<<-1;
else
   g<<i-1;

f.close();
g.close();
return 0;
}