$1-2+3-4+...$

The series above diverges, of course. Its **Abel sum** is defined as

$A=\lim_{x\to 1^-}\sum_{n=0}^{\infty}(-1)^{n+1} n x^n$

Find $A$ to three significant figures. Enter 666 if you come to the conclusion that no such Abel sum $A$ exists.

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