TeX source:

\mathrm{r}=\frac{\sum_{\mathrm{XY}-} \frac{\sum \mathrm{x} \sum \mathrm{Y}}{\mathrm{N}}}{\sqrt{\left(\sum \mathrm{X}^{2}-\frac{\left(\sum \mathrm{x}\right)^{2}}{N}\right)} \sqrt{\left(\sum \mathrm{Y}^{2}-\frac{\left(\sum \mathrm{Y}\right)^{2}}{N}\right)}}