(Challenge) Chef with Orchestra in the Kitchen

Chef has a large orchestra in his kitchen and he is preparing for a concert next week. He also has a music score, which is represented by a string $S$ with length $N$. Each note in the music score is represented by a lowercase or uppercase English letter; the values of lowercase letters 'a' through 'z' are $1$ through $26$ and the values of uppercase letters 'A' through 'Z' are $27$ through $52$.