Count Unique Alphabetic Characters

You are given a string. Your task is to count the number of unique alphabetic characters present in the string, ignoring the case of the letters. Non-alphabetic characters, including digits, punctuation, and spaces should be ignored.

Formally, given a string \( S \), you need to determine the number \( N \) of distinct characters in \( S \) that satisfy \( \text{isalpha}(c) = \text{true} \) (when considering the lowercase form of each character).

Examples:

• Input: "Hello, World!" → Output: 7
• Input: "12345!!!" → Output: 0
• Input: "H e l l o" → Output: 4

Note that the comparison is case-insensitive (i.e. 'A' and 'a' are considered the same character).

inputFormat

The input consists of a single line containing a string \( S \) which may include spaces, punctuation and other symbols.

outputFormat

Output a single integer representing the number of unique alphabetic characters present in the input string.

Hello, World!

7