Helping Dustin

Dustin, is the head of an Intelligence agency. He wants to send a secret message $S$ to his colleague.The message is a single word consisting of only lowercase english letters but he decides to encrypt the message for security reasons. He makes a string $M$ of length $N$, such that after deleting a substring of non-zero length from $M$, the remaining string is $S$. Calculate the number of all suc