Moving Average

Design a data structure that supports a stream of numbers and, after each new number $x_t$ arrives, returns the average of the most recent $k$ values:

${\text{MA}}_t=\frac{1}{k}{\sum }_{i=t-k+1}^t{x}_i.$

How do you make each update $\mathcal{O}(1)$ instead of $\mathcal{O}(k)$?