When converting a signed value to an unsigned value (and if the value cannot be represented with the unsigned type), the standard says: Add UINT_MAX + 1 until y

When converting a signed value to an unsigned value (and if the value cannot be represented with the unsigned type), the standard says: Add UINT_MAX + 1 until you get a valid unsigned value. Adding UINT_MAX + 1 to -1 gives you UINT_MAX.