Among the 26 male students in a class, 13 like to play basketball, 9 like to play football, and 12 like to play volleyball. And there are two male students who like to play both basketball and football, and another two male students who like to play both volleyball and football, but none of them likes all three kinds of balls. Ask how many male students like to play both basketball and volleyball.
某班26个男同学中有13个人喜欢打蓝球，9人喜欢踢足球，12人喜欢打排球．并且有两个男同学既喜欢打蓝球又喜欢踢足球，另外有两个男同学既喜欢打排球又喜欢踢足球，但没有一个男同学三种球都喜欢的．问有多少男同学既喜欢打蓝球又喜欢打排球．
<gadget id="calculator">12 - 2 - 2 - 26</gadget>
<output>-18</output>

<gadget id="calculator">13 + 9 + (-18)</gadget>
<output>4</output>

<result>4</result>