a carry - select adder structure comprising a carry generation network and a multiplexer to select a particular pre - calculatad sum of a bit - group via orthogonal signal levels of a hot - carry signal provided by said carry generation network , wherein in order to provide orthogonal signal levels of said hot - carry signal , the carry generation network comprises two carry lookahead trees working in parallel to each other , wherein a first carry lookahead tree provides a first signal level of the hot - carry signal , and a second carry lookahead tree provides a second , compared to the first signal level inverse signal level of the hot - carry signal . furthermore a method to operate such a carry - select adder is described .