import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner sc =new Scanner(System.in);
		String s = sc.next();
		//判断a长度不为1 
		while (s.length()>1) {
			//则每位相加
			int sum =0;
			for (int i = 0; i < s.length(); i++) {
				sum+=s.charAt(i)-48;
			}
			s=sum+"";
		}
		System.out.println(s);
	}

}