You probably divided the radius by the arc length and you forgot to convert units.

You need to use the formula $\displaystyle\theta=\frac{{s}}{{r}}\usepackage{cmbright}\Huge\$ , dividing the arc length by the radius. These two measurements must be in the same units.

The correct answer is: $\displaystyle\frac{\pi}{{2}}\usepackage{cmbright}\Huge\$