Two nonzero real numbers $a$ and $b$ satisfy $ab = a - b.$ Find all possible values of $\frac {a}{b} + \frac {b}{a} - ab.$

Enter all possible values, separated by commas.
<|im_start|>system
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<|im_start|>user
Two nonzero real numbers $a$ and $b$ satisfy $ab = a - b.$ Find all possible values of $\frac {a}{b} + \frac {b}{a} - ab.$

Enter all possible values, separated by commas.
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
2
Two nonzero real numbers $a$ and $b$ satisfy $ab = a - b.$ Find all possible values of $\frac {a}{b} + \frac {b}{a} - ab.$

Enter all possible values, separated by commas.
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