#include<fstream.h>
long x;
ifstream f("factorial.in");
ofstream g("factorial.out");

int zero(int z)
 {
  int m,r=5,nr=1,y=0;
  while(nr<z)
   {
    r=r+5;
    m=r;
    if(m%5==0) {m=m/5;nr++;}
    if(m%5==0) while(m%5==0) { m=m/5; nr++; y++; }
   }
  if(nr>x) return -1;
  else return z-y;
 }

int main()
 {
  f>>x;
  if(zero(x)<0) g<<zero(x);
  else g<<5*zero(x);
  return 0;
 }