Spell Shortening

Chef is attempting to learn magic, but it really hasn't been going very well. In order to pass an upcoming exam, Chef decides to just memorize magic spells - but even that turns out to be too hard! A *magic spell* is, quite simply, a string that consists of only lowercase Latin letters (i.e, the characters $\{\texttt{a,b,c}\ldots,\texttt{z}\}$). Chef has with him a magic spell $S$, and is stru