Given $${{a}^{2}}-{{b}^{2}}=\left( a+b \right)\times \left( a-b \right)$$.  Calculate: $$2{{0}^{2}}-{{19}^{2}}+{{18}^{2}}-{{17}^{2}}+{{16}^{2}}-{{15}^{2}}+\ldots \ldots +{{2}^{2}}-{{1}^{2}}=$$~\uline{~~~~~~~~~~}~．     Johnny spent the whole afternoon working out the sum.  $20$\textsuperscript{$2$}~- $19$\textsuperscript{$2$}~= $39$  $18$\textsuperscript{$2$}~- $17$\textsuperscript{$2$}~= $35$  $16$\textsuperscript{$2$}~- $15$\textsuperscript{$2$}~= $31$  $$\cdots $$  After a while, he gave up.  Is there a much faster way to work this out? Please help Johnny! 
Overseas Competition->Knowledge Point->Word Problem Modules->Average Problems ->Using Formulas