public void actionPerformed(ActionEvent event) {
			if (!currentInput.equals("0") && !currentInput.contains("-")) {
				currentInput = "-" + currentInput;
				returnField.setText(currentInput);
			}
			else if (currentInput.contains("-")) {
				currentInput = currentInput.substring(1, currentInput.length());
				returnField.setText(currentInput);
			}
		}