tell me wht doesn't this program work? or at least tell me an example that my program doesn't give us the right answer

var
       l,n,k :integer;
       sum   :longint;

function c(a,b:integer):integer;
   var
       i,a1,b1,a1b1:integer;
   begin
       a1:=1;
       for i:=1 to a do
          a1:=a1*i;
       b1:=1;
       for i:=1 to b do
          b1:=b1*i;
       a1b1:=1;
       for i:=1 to a-b do
          a1b1:=a1b1*i;
       c:=a1 div (b1*a1b1);
   end;

function power(r,s:integer):integer;
   var
       j,rs:integer;
   begin
       rs:=1;
       for j:=1 to s do
           rs:=rs*r;
       power:=rs;
   end;



begin
     readln(n,k);
     l:=-1;
     while 2*l<=n do
     begin
           inc(l);
           inc(sum,(c(n-l,l)*power(k-1,n-l)));
     end;
     writeln(sum);
end.

Try with Big Numbers, no standart type will pass this problem as far as I remember,otherwise the idea I think is  clear