program frr;
var f,g:text;
    c,n,p,nr:int64;
begin
assign(f,'fact.in'); reset(f);
assign(g,'fact.out'); rewrite(g);
readln(f,p);
if p=0 then
  writeln(g,'1')
  else
   begin
     if p<0 then
       writeln(g,-1)
       else
       begin
nr:=p div 2;
  if p mod 2=0 then
    n:=nr*10
  else
    n:=nr*10+5;
    writeln(g,n);
    end;
    end;
close(f);
close(g);
end.