import acm.program.*;

public class DigitalRoot extends ConsoleProgram {
	public void run()  {

		int number_user = readInt("Enter a number: ");
		int sum = 0;

		if(number_user < 0) {
			println("Error");
		} else {
			while( number_user > 0 || sum >= 9) {

					sum += number_user % 10;
					number_user /= 10;



			}
			println("" + sum + "");
		}
	}


	public static void main(String[] args) {
		new DigitalRoot().start();
	}

}