请问为什么只得了四十分？

var
s:string;
n,i,x:integer;
begin
readln(n);
read(s);
for i:=1 to length(s) do
begin
x:=ord(s[i])-ord('a')+1;
x:=(x+n) mod 26;
if x=0 then x:=x+1;
s[i]:=chr((x-1)+ord('a'));
end;
write(s);
end.