class UnderageException extends Exception {
    public UnderageException(String message) {
        super(message);
    }
}

public class VotingEligibility {
    public static void checkEligibility(int age) throws UnderageException {
        if (age < 18) {
            throw new UnderageException("You must be at least 18 years old to vote in the USA.");
        } else {
            System.out.println("You are eligible to vote.");
        }
    }

    public static void main(String[] args) {
        try {
            checkEligibility(16); // Should throw UnderageException
        } catch (UnderageException e) {
            System.out.println("Error: " + e.getMessage());
        }

        try {
            checkEligibility(20); // Should be eligible
        } catch (UnderageException e) {
            System.out.println("Error: " + e.getMessage());
        }
    }
}