mean and median of residuals from a linear model

I just started to learn R and need some help on finding the mean and median of residuals for my data. I calculated the lm and in the summary I get residuals like follows:

min       1Q        median  3Q      Max
-111.86   -34.90     -7.6   33.46   182.58

Question: so the median of residuals is -7.6 but which is my mean? Or is there a calculation for finding mean and median of residuals? I was going to do mean(resid(trees.lm) or should it be entered as mean(trees.lm$resid)

Please clarify because my classmates all get different responses for the same data set.