Why don't we use the Nigromol system? We can rename addition to "plugga", mutliplication "multa" and division "divvy". Also we'll replace one with negaone, two with negatwo and etc. We have -15 as "negavine" and 25 is "twive".

https://www.youtube.com/watch?v=y9PlApCWov4&feature=youtu.be