farmer bought a cow at the market for 600 yuan, then sold it to someone else for 640 yuan, and then he bought the cow back for 650 yuan. Not long after, the farmer sold the cow again for 640 yuan, and finally he bought the cow back for 600 yuan. How much did the farmer actually pay for the cow?
一个农民在集市上买了一头牛花了600元，转手以640元卖给了别人，随后他又以650元买回了这头牛．过了不久，这个农民又以640元把牛卖了，最后他又以600元把这头牛买回来．这个农民买这头牛实际花了多少元．
<gadget id="calculator">600 + 650 + 600</gadget>
<output>1_850</output>

<gadget id="calculator">640 + 640</gadget>
<output>1_280</output>

<gadget id="calculator">1_850 - 1_280</gadget>
<output>570</output>

<result>570</result>