Given a string S, return the number of substrings that have only one distinct letter. Example 1: Input: S = "aaaba" Output: 8 Explanation: The substrings with one distinct letter are "aaa", "aa", "a", "b". "aaa" occurs 1 time. "aa" occurs 2 times. "a" occurs 4 times. "b" occurs 1 time. So the answer is 1 + 2 + 4 + 1 = 8. Example 2: Input: S = "aaaaaaaaaa" Output: 55 Constraints: 1 <= S.length <= 1000 S[i] consists of only lowercase English letters.