You probably used the correct formula $\displaystyle\theta=\frac{{s}}{{r}}\usepackage{cmbright}\Huge\$ , with s representing arc length and r representing the radius, but you forgot to convert one of them. In order to use this formula you need to have the two measurements in the same units. The correct answer is: $\displaystyle\frac{\pi}{{2}}\usepackage{cmbright}\Huge\$