private BigDecimal getOperand(CalculatorReader expressionReader) {
        DecimalFormatSymbols decimalFormatSymbols = new DecimalFormatSymbols();
        decimalFormatSymbols.setDecimalSeparator('.');
        NumberFormat NUMBER_FORMAT = new DecimalFormat("0,0", decimalFormatSymbols);
        ParsePosition position = new ParsePosition(0);
        Number number = NUMBER_FORMAT.parse(expressionReader.getCurrentExpression(), position);

        if (number != null) {
            BigDecimal result = new BigDecimal(number.doubleValue());
            expressionReader.incPosition(position.getIndex());
            //System.out.println("--getOperand = " + result);
            return result;
        }
        return null;
    }