Java 14 how to put numbers in a GUI

hallo i am using eclips and whant to put numbers into a GUI but it wont let me.

this is my code (it keeps telling me to change the double a and b into a string but then i cant do maths maby it is to do with the JOptionPane.showInputDialog ) :

import javax.swing.JOptionPane;

public class test4 {

	public static void main(String[] args) {
		
		double c;
		
		JOptionPane.showMessageDialog( null, "This will find out the hypotenuse of your triangle.");
		double a = JOptionPane.showInputDialog("Enter Side A:");
		double b = JOptionPane.showInputDialog("Enter Side B:");
		c = Math.sqrt((a*a)+(b*b));
		JOptionPane.showMessageDialog( null, "hypotenus:"+c);
		
	}

}