Guys I do nor know how to solve this problem about threads. I have read lots of sources but cannot find anything usefull

import java.util.Scanner;

class sum implements Runnable {
    double a, b;

    public sum(double a, double b) {
        this.a = a;
        this.b = b;
    }

    public void run() {
        add(a, b);

    }

    public void add(double a, double b) {
        double sum = 0;
        synchronized (this) {
            for (double i = a; i <= b; i++) {
                sum = sum + i;

            }
            System.out.println("Sum of " + a + " to " + b + " numbers = " + (double) sum);
        }

    }
}

public class Main {
    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();

        Thread t1 = new Thread(new sum((double) 1, (double) n / 4));
        Thread t2 = new Thread(new sum((double) (n / 4) + 1, (double) n / 2));
        Thread t3 = new Thread(new sum((double) (n / 2) + 1, (double) n * 3 / 4));
        Thread t4 = new Thread(new sum((double) (n * 3 / 4) + 1, (double) n));
        Thread main = new Thread(new sum((double) 1, (double) n));

        t1.start();
        t2.start();
        t3.start();
        t4.start();
        main.start();

        try {
            t1.join();
            t2.join();
            t3.join();
            t4.join();
            main.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

    }
}