Help computing Pi!

Hi. We have been asked to compute Pi with the formula:

PI = 3 + 4/(2*3*4) - 4/(4*5*6) + 4/(6*7*8) - 4/(8*9*10) + ...
Allowing the user to determine the amount of computations to perform(5 shown in the above formula)

I'm completely stumped at this stage and cannot get the correct answer no matter what I try. Does anyone have any idea where a problem is in this code?

Code:
```import javax.swing.JOptionPane;

import java.util.Scanner;

public class pi
{

public static void main(String[] args)
{

String digitInput = JOptionPane.showInputDialog( "How many terms would you like to be used in the computation?");

Scanner termScan = new Scanner (digitInput);

double terms = termScan.nextDouble();

double pistart = 3;
double termCount = 0;
double num1 = 2;
double num2 = 3;
double num3 = 4;
double four = 4;

while (termCount <= terms)
{

double piloop = (four/(num1 * num2 * num3));
double finalpi = pistart + piloop;

num1 = num1 + 2;

num2 = num2 + 2;

num3 = num3 + 2;

four = four * (-1);

termCount ++;

JOptionPane.showMessageDialog(null, + finalpi);

}
}
}```