import java.util.*;

public class Main {
    public static void main(String[] args){
        Scanner sc=new Scanner(System.in);
        int number=sc.nextInt();
        int length=String.valueOf(number).length();
        int checkNum=0;
        while(length>=2){
            for(int i=0;i<length;i++){
                checkNum+=number%10;
                number/=10;
                //System.out.println(checkNum+" "+length);
            }
            number=checkNum;
            length=String.valueOf(number).length();
            checkNum=0;
        }
        System.out.println(number);
    }
    
}