You are trying to use the variable a before it is declared. The forward referencing rule says that you cannot use a variable on the right hand side of an assignment statement or use it to print something before it can be declared.
Hope this helps.
SCJP 1.4, SCWCD 1.4 - Hints for you, Certified Scrum Master
Did a rm -R / to find out that I lost my entire Linux installation!