$$\underbrace {{\rm{a}}\,{\rm{ + }}\,{\rm{a}}\,{\rm{ + }}\,{\rm{a}}\,{\rm{ + }}\,.......\,{\rm{ + }}\,{\rm{a}}}_{{\rm{n}}\,{\rm{times}}} = {a^2}b$$ and $$\underbrace {b\,{\rm{ + }}\,b\,{\rm{ + }}\,b\,{\rm{ + }}\,.......\,{\rm{ + }}\,b}_{m\,\,{\rm{times}}} = a{b^2}$$, where a, b, n and m are natural numbers. What is the value of $$\left( {\underbrace {b\,{\rm{ + }}\,b\,{\rm{ + }}\,b\,{\rm{ + }}\,.......\,{\rm{ + }}\,b}_{n\,\,{\rm{times}}}} \right)\left( {\underbrace {a\,{\rm{ + }}\,a\,{\rm{ + }}\,a\,{\rm{ + }}\,.......\,{\rm{ + }}\,a}_{m\,\,{\rm{times}}}} \right)$$?