Find the maximum number of natural numbers $x_1,x_2, ... , x_m$  satisfying the conditions:
a) No $x_i - x_j , 1 \le i < j \le m$ is divisible by $11$,  and
b) The sum $x_2x_3 ...x_m + x_1x_3  ... x_m + \cdot \cdot \cdot + x_1x_2... x_{m-1}$ is divisible by $11$.