Convert to base 10

//chuyen so nguyen duong tu co so bat ki (<20) sang thap phan
 uses crt;

 var x:ansistring;
     b:byte;
     kitu: array [1..100] of byte;

 procedure xlkitu;
 var i:char; j:byte;
 begin
        j:=10;
        for i:='A' to 'J' do
           begin
                kitu[ord(i)]:=j;
                inc(j);
           end;
 end;

 function tp:int64;
 var kq:extended;
     i,n: byte;
     s:integer;
     code:integer;
 begin
        tp:=0; n:=0;
        for i:=length(x) downto 1 do
                begin
                        if x[i] in ['0'..'9'] then val(x[i],s,code)
                        else s:=kitu[ord(x[i])];
                        tp:=tp + s*round(exp(ln(b)*n));
                        inc(n);
                end;
 end;

 begin
        clrscr;
        xlkitu;
        write('Nhap x: ');readln(x);
        write('Nhap co so b: ');readln(b);
        writeln('x trong co so 10 la: ',tp);
        readln
 end.