Letters missing from a string

You are given a string. Which letters of the Latin alphabet do not occur in it at all? Uppercase and lowercase count as the same letter: A and a are one letter.

A single line — a string of $1$ to $1000$ ASCII characters.

The first line contains the number of missing letters. The second line contains those letters — lowercase, in alphabetical order, without spaces (if all letters occur, the second line is empty).

Collect the letters of the string (converted to lowercase) into a set, then print the letters from a to z that are not in it. The sentence in the second example contains all $26$ letters — the answer is $0$.