What Is This, a Crossover Episode

You are given an integer $D$. You have to find a string $S$ such that the following conditions are satisfied: - $S$ is non-empty and its length does not exceed $7 \cdot D$. - $S$ contains only lowercase English letters. - The number of distinct substrings in $S$ minus the number of palindromic substrings in $S$ equals $D$. Here, when counting palindromic substrings, a substring that occurs mult