The Lens Maker's formula is $\frac{1}{f} = (n_{rel}-1)(\frac{1}{R_1}-\frac{1}{R_2})$, where $n_{rel}$ is the refractive index of the lens relative to the surrounding medium ($n_{rel} = n_{lens}/n_{medium}$). Remember the sign convention for radii of curvature (light travels left to right, centers of curvature to the right are positive, to the left are negative).