Помогите в Паскале!!

Var s:string; Os, k, r, l, c:integer;
Begin
readln (s);
readln (Os);

k:=1;r:=0;
l:=length (s);
while l>0 do Begin
Case s of '0':c:=0;
'1':c:=1;
'2':c:=2;
'3':c:=3;
'4':c:=4;
'5':c:=5;
'6':c:=6;
'7':c:=7;
'8':c:=8;
'9':c:=9;
'A':c:=10;
'B':c:=11;
'C':c:=12;
'D':c:=13;
'E':c:=14;
'F':c:=15;
else write ('r:=r+c*k; k:=k*Os; l:=l-1');
end;
writeln(r);
end.

Что здесь не так?