$$\frac{1}{{\left( {\sqrt 9 - \sqrt 8 } \right)}} \, - $$ $$\frac{1}{{\left( {\sqrt 8 - \sqrt 7 } \right)}} \, + $$ $$\frac{1}{{\left( {\sqrt 7 - \sqrt 6 } \right)}} \, - $$ $$\frac{1}{{\left( {\sqrt 6 - \sqrt 5 } \right)}} \, + $$ $$\frac{1}{{\left( {\sqrt 5 - \sqrt 4 } \right)}}$$ is equal to ?