import java.io.*;
import java.util.*;

interface AdvancedArithmetic{
   int divisorSum( int n );
}


class Calculator implements AdvancedArithmetic {
    
    public int divisorSum( int n ) {
        
        int sum = 1;
        // 1 and n are always divisors
        // Greatest divisor <= n / 2
        for ( int i = 2; i <= n / 2; ++i ) {
            if ( n % i == 0 ) {
                sum += i;
            }
        }
        
        return sum + n;
        
    }
    
}

public class Solution {

    public static void main( String[] args ) {

        Scanner sc = new Scanner( System.in );
        int n = sc.nextInt();
      	AdvancedArithmetic myCalculator = new Calculator(); 
        int sum = myCalculator.divisorSum( n );
        System.out.println("I implemented: AdvancedArithmetic\n" + sum);
    	sc.close();
    	
    }

}

Add day 19 solution