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The metric tensor and its inverse are diagonal: $g_{xx}=1$, $g_{yy}=f^2$, $g^{xx}=1$, $g^{yy}=f^{-2}$ on a patch where $f\ne 0$. Only ${\partial }_x{g}_{yy}=2f{f}^{\prime }$ is nonzero. The Christoffel symbol formula therefore giveswith all other components zero. Using the stated Riemann curvature tensor convention,so $\boxed{R^x{}_{yxy}=-f{f}^{\prime \prime }}$. This is the curvature of a two-dimensional warped metric; its Gaussian curvature is $-f^{\prime \prime }/f$.