#include <iostream.h>
#include <math.h>
#include <fstream.h>
int factorial(unsigned long int n)
{
    int i=0;
    unsigned long int nr=0;
    while(pow(5,i)<n)
    i++;
    for(int k=1;k<=i;k++)
    nr+=n/pow(5,k);
return nr;
}
void binar(unsigned long int k,unsigned long int j,unsigned long int nr)
{
    fstream g("fact.out",ios::out);
    int ok=0;
    unsigned long int cautat=0;
    while(k<=j&&ok==0)
    {
                      if(factorial(k)==nr&&ok==0)                        
    {
                                                ok=1;
                                                cautat=k;
                                                }
                                                k++;
                                                }
if(ok==0)
g<<-1;
else
g<<cautat;
}
int main()
{
    unsigned long int nr,k,j;
    fstream f("fact.in",ios::in);
    fstream g("fact.out",ios::out);
    f>>nr;
    k=nr*4;
    j=nr*5;
    if(nr==1)
    g<<0;
    else
    if(nr/5==((nr%5)+1))
    g<<-1;
    else
    binar(k,j,nr);
    return 0;
}