#include <cstdlib>
#include <iostream>

typedef unsigned long int uli;

using namespace std;

uli cp[] = {0,5,25,125,625,3125,15625,78125,390625,1953125,9765625,48828125,244140625};

uli Ev(uli p)
{
int i=2;
uli rez=5*p;
while(cp[i]<=5*p && i<=12)
{
  rez-=5*(i-1);
  if(cp[i]*2<=5*p)rez-=5*(i-1);
  i++;
}
return rez;
}


int main(int argc, char *argv[])
{
   
    uli pp;
    freopen("fact.in","r",stdin);
    cin >> pp;   
    fclose(stdin);
    freopen("fact.out","w",stdout);
    if(Ev(pp)==Ev(pp+1)) cout<<-1;
    else cout<<Ev(pp);
    return EXIT_SUCCESS;
}