Rút gọn B

B=$\frac{-1}{2}+\frac{-1}{6}+\frac{-1}{12}+\frac{-1}{20}+\frac{-1}{30}+\frac{-1}{42}+\frac{-1}{56}+\frac{-1}{72}+\frac{-1}{90}$