求助、求助：为何40？

我是这么写的，哪位大神帮忙看一下？

var
  i,k,n,j:longint;
s: string;
begin
  readln(n);
readln(s);
for j:=1 to length(s) do
  s[i]:=chr(ord(s[i])+n);
  for i:=1 to length(s)do
    begin
    k:=ord(s[i]);
   if ((k<65)and(k>90))or((k<97)and(k>122))then s[i]:=chr(ord(s[i])-26);
   end;
writeln(s);
end.