In a survey, 'Non-response Bias' is defined as $B = \frac{N_{nr}}{N} (\bar{Y}_r - \bar{Y}_{nr})$. If $N=1000$ and $N_{nr}=200$ units fail to respond, and the sample mean of respondents is $\bar{Y}_r = 50$ while the estimated mean of non-respondents is $\bar{Y}_{nr} = 40$, what is the bias $B$?