Username Validation

You are given a username string and need to determine whether it is valid according to the following criteria:

• The username must be between 6 and 20 characters long.
• It can only contain letters (both uppercase and lowercase), digits, and underscores.
• The username must start with a letter.

Your task is to read the username from stdin and print Valid if it meets the criteria, or Invalid otherwise.

Note: If a formula is necessary, it is given in the LaTeX format. For example, the valid length condition can be written as: \(6 \leq |username| \leq 20\).

inputFormat

The input consists of a single line containing the username string.

username

outputFormat

Output a single line: Valid if the username satisfies all the conditions, or Invalid otherwise.

John_Doe123

Valid