# Question: Write a Python program to check if a character is an uppercase or lowercase letter.

def main(char: str) -> None:
    if char.isupper():
        print(f"{char} is an uppercase letter.")
    elif char.islower():
        print(f"{char} is a lowercase letter.")
    else:
        raise ValueError("Invalid input. Please enter an alphabetic character.")

if __name__ == '__main__':
    main('A')
    main('a')
    main('B')
    main('b')
    main('1')  # also works with a int