How is the net intracavity dispersion of a fiber laser calculated?

It is known that the erbium-doped fiber is 19.9 m long with a dispersion coefficient D of –30 ps/(nm·km), and the single-mode fiber is 3.7 m long with a dispersion coefficient D of 17 ps/(nm·km). The total intracavity dispersion is 0.68 ps². I found a formula that is D·L·pulse spectral width, but when I substitute the values into it, the calculation is incorrect, and the units also seem to be wrong.