Increment decrement expression

b=a++ + --a +a++ + ++a; i m getting 6 after evaluating but the output is 5. How??

Since the increment/decrement operator modifies its operand, use of such an operand more than once within the same expression can produce undefined results. For example, in expressions such as a++ + --a , it may not be clear to a user in what sequence the subtraction and increment operations should be performed. Such expressions generally invoke undefined behavior, and should be avoided.

Refer this, explanation with further links for reference reading are there.