var ga,i,n:longint;
    b:boolean;
begin
  read (n);
  while i=n do
  begin
    if n mod i = 0 then
    begin
  ga:=i
b:=true;
break;
end;
inc (i);
end;
if (ga=2)and (not(b))then
  writeln (n div 2 - 1)
else
  writeln (ga-1);
end.