Let $S_1$ be a square of side $a$. Another square $S_2$ is formed by joining the mid-points of the sides of $S_1$. The same process is applied to $S_1$ to form yet another square $S_3$, and so on. If $A_1$, $A_2$, $A_3$, \dots are the areas and $P_1$, $P_2$, $P_3$, \dots are the perimeters of $S_1$, $S_2$, $S_3$, \dots, respectively, then the ratio $\frac{P_1 + P_2 + P_3 + \dots}{A_1 + A_2 + A_3 + \dots}$ equals: