树根~~~

数根__牛客网

import java.util.*;
public class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        while(sc.hasNext()){
            String str = sc.nextLine();
            
            while(str.length() > 1){
                int sum = 0;
                for(int i = 0;i < str.length();i++){
                    sum+=str.charAt(i)-'0';
                }
                str = String.valueOf(sum);
            }
            System.out.println(str);
        }
    }
}