1109: 数根（函数专题）

import java.util.Scanner;
public class Main {
    
    
    public static int digitSum(int n)
    {
    
    
        int res=0;
        while(n>0)
        {
    
    
            res+=n%10;
            n=n/10;
        }
        return res;
    }
    public static void main(String[] args) {
    
    
        Scanner cin = new Scanner(System.in);
        int n=cin.nextInt();
        while(n>=10)
            n=digitSum(n);
        System.out.println(n);
    }
}