Standard Deviation Of Difference In Sample Proportions

To compare the proportion of adults who exercise regularly in two cities, a researcher selected independent random samples. The sample from City A consisted of 400 adults, of whom 45% exercise regularly. The sample from City B consisted of 600 adults, of whom 38% exercise regularly. What is the standard deviation of the sampling distribution for the difference in sample proportions ($\hat{p}_A - \hat{p}_B$)?