This Program Got AC within 0.01 Sec!
----------------------------------------------------------------------
program answer1054;
const
     st:array[0..1,0..2]of 1..3=((1,2,3),(1,3,2));
var
   d:array[1..31]of 1..3;
   e:array[1..31]of 0..1;
   n,i,s,f,delta,j,s1:longint;
function mi2(x:integer):longint;{mi2=2^x}
         var
            s:longint;
            i:integer;
         begin
              s:=1;
              for i:=1 to x do s:=s*2;
              mi2:=s;
         end;
function c(x:integer):longint;{N-dished Hanoi tower step uses}
         begin
              if x=31 then c:=maxlongint else c:=mi2(x)-1;
         end;
begin
     readln(n);
     for i:=1 to n do read(d[i]);
     s:=0;f:=c(n);
     fillchar(e,sizeof(e),0);
     for i:=n downto 1 do begin
         s1:=1;delta:=0;
         for j:=i+1 to n do begin
             delta:=delta+e[j]*s1;
             s1:=s1*2;
         end;
         if d[i]=st[(n+i) mod 2,delta mod 3] then begin
         f:=f-mi2(i-1);
         end else
            if d[i]=st[(n+i) mod 2,(delta+1) mod 3] then
               begin
                    s:=s+mi2(i-1);
                    e[i]:=1;
               end else begin
                   writeln(-1);
                   halt;
               end;
     end;
     writeln(s);
end.