Clarification on the Calculation "right - left - 1" in Palindrome Substring Code

Question

I am working on a Python function to find the longest palindrome substring within a given string. I came across a code snippet that utilizes the expression "right - left - 1" to determine the length of the palindrome. While the code works effectively, I am seeking a deeper understanding of why this specific calculation is used.

Here is the relevant code snippet:

def longestPalindrome(self, s):
    if not s:
        return ""

    start = 0
    end = 0

    for i in range(len(s)):
        len1 = self.expand_around_center(s, i, i)
        len2 = self.expand_around_center(s, i, i + 1)
        max_len = max(len1, len2)

        if max_len > end - start:
            start = i - (max_len - 1) // 2
            end = i + max_len // 2

    return s[start:end+1]

def expand_around_center(self, s, left, right):
    while left >= 0 and right < len(s) and s[left] == s[right]:
        left -= 1
        right += 1

    return right - left -1   # this is the line in question

I would greatly appreciate it if someone could provide insights into why right - left - 1 accurately represents the length of the palindrome in this context. Understanding the rationale behind this calculation will contribute significantly to my comprehension of palindrome substring algorithms.