Given a string s, return its total appeal, which is calculated by summing the appeals of all its substringsA substring is a contiguous sequence of characters within a string..

The appeal of a string is defined as the count of unique characters present in that string.

For instance, the appeal of “xyzxz” is $3$, as it contains three distinct characters: ‘x’, ‘y’, and ‘z’.

Constraints:

• $1 \leq$ s.length $\leq 10^3$

• s consists of only lowercase English letters.