The four-fold agreement measure

For 52 weeks, an ice cream shop tracked whether the mean temperature and ice cream sales rose or fell compared with the previous week. The result:

In 38 weeks ($21 + 17$) both variables moved in the same direction, in 14 weeks ($8 + 6$) in opposite directions.

Compute the four-fold measure $(\text{concordant} - \text{discordant}) / n$ to two decimal places.