Given the integers zero, one, low, and high, we can construct a string by starting with an empty string, and then at each step perform either of the following: This can be performed any number of times. A good string is a string constructed by the above process having a length between low and high (inclusive). Return the number of different good strings that can be constructed satisfying these properties. Since the answer can be large, return it modulo 109 + 7.