Given two strings s and t, return the number of pairs of substrings that differ by exactly $1$ character.

A pair of substrings $(s_{sub}, t_{sub})$ is valid if $s_{sub}$ is a substring of s, $t_{sub}$ is a substring of t, they have the same length, and they differ at exactly $1$ position.

Note: A substring is a contiguous sequence of characters within a string.

Constraints:

• $1 \leq$ s.length, t.length $\leq 100$

• s and t consist of lowercase English letters only.

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