The expression $$\frac {1}{ 1 \sqrt{2} + 2 \sqrt{1} }+ \frac {1}{ 2 \sqrt{3} + 3 \sqrt{2} } + \ldots + \frac {1} { 9999 \sqrt{10000} + 10000 \sqrt{9999} }$$ is equal to $$\frac { a}{b}$$, where $$a$$ and $$b$$ are coprime positive integers. What is $$a + b$$?