program factorial;
var f,g:text;
    c,p,i,ci:longint;
begin
assign(f,'fact.in');
assign(g,'fact.out');
reset(f);
rewrite(g);
read(f,p);
c:=p*5;
for i:=1 to p-1 do begin
  ci:=i;
{  if (i mod 5=0)then c:=c+5; }
  {while (ci mod 5=0)and(ci>0)do
    begin
      c:=c-5;
      ci:=ci div 5;
    end;   }
  if (ci mod 5=0)then c:=c-5;
  end;
write(g,c);
close(f);
close(g);
end.