cifre

i don't understand what's wrong with my code:

program cifre;
var
   n,p,c,d,k : longint;
function ex(a,b : integer) : longint;
var
   rez : longint;
begin
   rez := 1;
   while b > 0 do
      begin
         if odd(b) then rez := rez*a;
         b := b div 2;
         a:= sqr(a);
      end;
   ex := rez;
end;
 
begin
   readln(n);
   p := 9;
   c := 1;
   while n > p do
      begin
         n := n-p;
         p := p*10+9*ex(10,c);
         c := c+1;
      end;
   if n mod c = 0 then
      d := n div c -1
   else d :=  n div c;
   d := d+ex(10,c-1);
 
   if n mod c = 0 then writeln(d mod 10)
   else
      begin
         k := n mod c;
         while k > 0 do
            begin
               d := d div 10;
               k := k-1;
            end;
         writeln(d mod 10);
      end;
 
end. 

It only passes 2/5 test cases...