I've written program that finds the number of steps but Can't
determine whether it's possible or not.

var
   N:integer;
   s,d:array[1..31] of integer;
   i,j,t:integer;
   K:longint;
   n2:array[0..30] of longint;

begin

   readln(N);
   for I:=1 to n do read(s[i]);
   n2[0]:=1;
   for I:=1 to 30 do n2[i]:=n2[i-1]*2;
   for I:=1 to n do d[i]:=1;

   k:=0;
   for I:=n downto 1 do
   begin
      IF D[I]<>S[I] then
      begin
          K:=K+N2[i-1];
         for j:=1 to 3 do if (s[i]<>j) and (d[i]<>j) then
         begin
            t:=j;
            break;
         end;
         for J:=1 to i-1 do if d[j]=d[i] then
         begin
            d[j]:=t;
         end;
         d[i]:=s[i];
         end;
   end;
    writeln(k);

end.