For a system with two levels of cache, define:
- $T_{c_1}$ is the first-level cache access time
- $T_{c_2}$ is the second-level cache access time
- $T_m$ is the memory access time
- $H_1$ is the first-level cache hit ratio
- $H_2$ is the combined first/second level cache hit ratio
Provide an equation for the average access time $T_a$ of a read operation.