Jumping flea

Level pending

There are two points on the x-axis: $$A=39$$ and $$B=-18$$ A flea is on the $$0$$ point of the x-axis. Every second the flea jumps to the right or to the left of $$1$$ unit with equal probability. The flea stops when it reaches $$A$$ or $$B$$. How many seconds it takes on average to stop?

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