A stamp merchant bought a rare stamp for 700 yuan, sold it for 800 yuan, and bought it back for 900 yuan. Finally sold it for 1000 yuan. So how much money did he earn from these few transactions?
一个邮票商人用700元买一张罕见的邮票，以800元卖掉了，又用900元买回来．最后以1000元卖掉了．那么从这几次买卖中他赚了多少元．
<gadget id="calculator">800 - 700</gadget>
<output>100</output>

<gadget id="calculator">1_000 - 900</gadget>
<output>100</output>

<gadget id="calculator">100 + 100</gadget>
<output>200</output>

<result>200</result>