program factorial;
var p,i:longint;
    rez:real;
    f,g:text;
    cond:boolean;
    aux:int64;
begin
assign (f,'fact.in'); reset (f);
assign (g,'fact.out'); rewrite (g);
read (f,p);
aux:=5;
cond:=false;
for i:=2 to 13 do begin if (aux=p) then begin cond:=true;
                                              break;
                                        end;
                        aux:=aux*5;
                  end;
if (p<>0) and (cond<>true) then begin rez:=int(5*(p-int(ln(5*p)/ln(5)-1)));
                                      write (g,rez:10:0);
                                end
                           else if (p=0) then write (g,'1')
                                         else if (cond) then write (g,'-1');

close (f);
close (g);
end.