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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