import re

def is_valid_password(password):
    if (6 <= len(password) <= 12 and
        re.search(r'[a-z]', password) and
        re.search(r'[0-9]', password) and
        re.search(r'[A-Z]', password) and
        re.search(r'[$#@]', password)):
        return True
    return False

def main():
    password = input("Enter a password to check its validity: ")
    if is_valid_password(password):
        print("Password is valid.")
    else:
        print("Password is invalid. It must contain:")
        print("- At least 1 letter between a and z")
        print("- At least 1 number between 0 and 9")
        print("- At least 1 letter between A and Z")
        print("- At least 1 character from $, #, @")
        print("- Minimum length of 6 characters")
        print("- Maximum length of 12 characters")

if __name__ == "__main__":
    main()