By definition, there is no such thing as a negative unsigned integer.
When you go beyond the lower limit of an unsigned integer, the value "wraps" starting at the maximum possible value. (The same thing happens the other way around).
This mechanism also starts when converting a negative "signed" value to an unsigned one.
, -1 $maximumUnsignedValue. -$maximumSignedValue $maximumUnsignedValue - $maximumSignedValue + 1.