Chef and Secret Password

Chef wants to prepare a special dish. There is a special ingredient Chef needs for this dish, so he decided to search for it in all of Chefland. After some research, he discovered it in a secret cave. He may only enter the cave if he has a password; fortunately, a string $S$, which serves as a hint for this password, is written next to the cave. Chef knows that the secret password is a non-empty