Computing Pi Using Leibniz Series what is delta(difference in running computation)?

Question

I have to compute pi to six decimal precision using the leibniz series and I managed to do it except the assigment has some restrictions.

pi / 4 = 1 -1/3 + 1/5 - 1/7 ....

" cannot utilize the math library π in your computation - direct or indirect. Use ONLY the delta (difference) of your running computation to determine when to stop your loop "

I do not understand what this means by difference of your running computation. Another way I was told was "When your computed value ceases to change then your loop can stop"

package com.company;

public class Main {

public static void main(String[] args) {

    double series = 0;
    double denominator = 1;
    double numerator = 1;
    double pi;
    double testingPi;
    double formattedTestingPi = 0;
    double formattedMathPi = Math.round(Math.PI * 1000000.0) / 1000000.0;
    int max = 1200000;
    int iterations = 0;

    for(int i = 1; i < max;i++)
    {
        if((i % 2) != 0)
        {
            series = series + (numerator/denominator);
        }
        else if((i % 2) == 0)
        {
            series = series + ((numerator/denominator) * -1);
        }

        denominator = denominator + 2;
        testingPi = series * 4;

        formattedTestingPi = (Math.round(testingPi * 1000000.0))/1000000.0;

        if( formattedTestingPi == formattedMathPi)
        {
            iterations = i;
            i = max;
            System.out.println("We stop");
        }
    }

    pi = series * 4;

    System.out.println("Number of Iterations       :" + iterations);
    System.out.println("Unformatted Series         :" + series);
    System.out.println("Unformatted Math Library PI:" + Math.PI);
    System.out.println("Unformatted Computed     PI:" + pi);
    System.out.println("Formatted Computed       PI:" + formattedTestingPi);
    System.out.println("Formatted Math Library   PI:" + formattedMathPi);

 }

}

I do not want the solution to the assignment, I just want to know what does delta of computation mean and how is it different what I'm doing right now?

Output

Number of Iterations :1181461

Unformatted Series :0.7853983749998679

Unformatted Math Library PI:3.141592653589793

Unformatted Computed PI:3.1415934999994715

Formatted Computed PI:3.141593

Formatted Math Library PI:3.141593