fix coefficient sign error

src/reducer.c

@@ -19,9 +19,9 @@ static double get_all_terms_with_exponent(s_term *terms, int exponent)
         }
 
         // sign
-        sign = terms[i].sign == TERM_PLUS ? 1 : -1;
+        sign = 1;
         if (terms[i].position == TERM_RIGHT)
-            sign *= -1;
+            sign = terms[i].sign == TERM_PLUS ? -1 : 1;
 
         // add coefficients
         coefficient += terms[i].coefficient * sign;