Rule of sum and rule of product

Suppose that an action consists of $k$ steps:

• the first step can be performed in $n _1$ ways
• the second step can be performed in $n _2$ ways (regardless of how the previous step was performed)
• ...
• the $k$th step can be performed in $n_k$ ways (regardless of how the previous steps were performed)

How many ways can the action be performed?