问题 E: Strings, Subsequences, Reversed Subsequences, Prefixes

For a string $S=S_0S_1...S_{n-1}$ of length $n$, its reversed string is defined as $R(S)=S_{n-1}S_{n-2}...S_{0}$. Chino has two strings $S$ and $T$, she wants to know how many **distinct** non-empty subsequences $seq$ of $S$ such that $T$ is simultaneously a prefix of both $seq$ and $R(seq)$. Since the answer may be large, please output the result modulo $10^9+7$. Some definitions are seen in the notes for explanation.

The first line contains two positive integers $N$ and $M$ $(1\leq N,M\leq 10^6)$ --- the lengths of $S$ and $T$. Then the next two lines contain the contents of strings $S$ and $T$ $(S_i,T_i \in [a,z])$.

Only one line containing a non-negative integer --- the answer to the problem modulo $10^9+7$.

seq can be "aba","abba","ababa","abbba","abaaba","ababba","abbaba","abababa". A non-empty subsequence of string $S$ is defined as a new string obtained by deleting one or zero or more characters from the string (while keeping at least one character). The prefix of string $S$ is defined as the part left after deleting all characters from a certain position to the end of the string. Two strings are **distinct** iff their lengths or contents are different.