here is my code:
I don't know wht's the error ,anyone help me?

Const
  u=['a'..'z']+['A'..'Z'];
Var
  s:string;
  ch:char;
  i:integer;

Begin
  read(ch);
  while not eof do
     begin
       s:='';
       while (ch in u) and (ord(ch)<>26) do
         begin
           s:=s+ch;
           read(ch);
         end;
       for i:=length(s) downto 1 do write(s[i]);
       while (not (ch in u)) and (ord(ch)<>26) do
         begin
           write(ch);
           read(ch);
         end;
    end;
End.

check whether you output EOF. If you output it you will get WA. I got
really angry when I understood that my program didn't work, because I
print EOF!!!!

what is EOF?