Xiaohong went to the bookstore to buy a fairy tale book worth 7.8 yuan, but it was 0.7 yuan short, so she had to buy a notebook for 5.1 yuan, and the remaining money just bought a pen. How much is this pen?
小红到书店想买一本7.8元的童话书，可是差0.7元，她只好买了5.1元的笔记本一本，剩下的钱正好买了一枝钢笔，这枝钢笔多少钱？
<gadget id="calculator">7.8 - 0.7 - 5.1</gadget>
<output>2</output>

<result>2</result>