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Look at the tutorials:
Assignment, Arithmetic, and Unary Operators (The Java™ Tutorials > Learning the Java Language > Language Basics)

also ask google

There is a shortcut operator (+=) that you can use to add one variable to another:
theTotal += aDetail;
instead of
theTotal = theTotal + aDetail;

Define a separate variable to hold the sum of the numbers.
Add each number that is read to that variable.