Big round meeting table !

A factory has $2^n$ workers . Every monday there is ameeting held and all of them sit on a big circular table. All the workers are divided into $a_1$ type groups each of 2 workers who should sit near each other . they are again divided into $a_2$ type groups each consisting of 2 $a_1$ type groups which should sit adjacent to each other . this is continued to form $a_3$ , $a_4$ ... type groups .

Find $2^{n+1}$ -(no of ways the workers can sit on the table)