Lập trình Pascal: Thao tác với chuỗi ký tự

Program DEM_SO_KY_TU;
Uses Crt;
Var st:string[30];
       x:char;i,dem:integer;
Begin
      Clrscr;
      Writeln('DIEM SO LAN XUAT HIEN CUA 1 KY TU');
      Writeln('------------------------------------------------------');
      Write('Nhap chuoi ky tu la:');readln(st);
      Write('Nhap ky tu x=');readln(x);
      dem:=0;
      For i:=1 to length(st) do
      If x= st[i] then dem:=dem+1;
           Writeln('so lan xuat hien cua ',x,' trong chuoi "', st,'" la: ',dem);
      Readln;
End.

Program CHUOI_DOI_XUNG;
Uses Crt;
Var st:string;
     i:integer;
     ok:boolean;
Begin
     Clrscr;
    Writeln('KIEM TRA CHUOI DOI XUNG');
    Writeln('----------------------------------------');
     Write('Nhap chuoi ky tu: ');Readln(st);
     ok:= true;
For i:=1 to length(St)div 2 do
If st[i]<>st[length(st)-i+1] then
     ok:=false;
If ok then
      Writeln(st,' la chuoi doi xung')
Else
      Writeln(st, ' khong la chuoi doi xung');
Readln;
End.