How is overflow detected in two’s complement?

I see that when I subtract positive and negative number using two's complement I get overflows. For example, if I subtract 1 from 2 I get:

2 = 0010
1 = 0001 -> -1 = 1111
2 + (-1) -> 0010 + 1111 = 10001

So here the result has fifth left bit 10001 – is it overflow? I’ve found these rules for detected overflows with two’s complement:

If the sum of two positive numbers yields a negative result, the sum has overflowed. If the sum of two negative numbers yields a positive result, the sum has overflowed. Otherwise, the sum has not overflowed.

Can anyone please elaborate on these and show example?

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