function nr5(a:longint):word;
var i:integer;
begin
  i:=0;
  while a mod 5=0 do begin inc(i);a:=a div 5; end;
  nr5:=i;
end;

var p,n,nr0,i:longint;
begin
assign(input,'fact.in');reset(input);
assign(output,'fact.out');rewrite(output);
  readln(p);
  if p=0 then write('1') else begin
    n:=1;
    while n*5<p*5 do begin n:=n*5;inc(i);end;
    write(p*5-(5*(i*(i-1)div 2)));
  end;
close(input);close(output);
end.