Among the foreign language teachers in a certain area, 35 know English, 34 know French, and 21 know both languages. How many foreign language teachers are there?
某地区的外语教师中，有35人懂英语，34人懂法语，两种语言都懂的有21人，共有多少个外语教师．
<gadget id="calculator">35 - 21</gadget>
<output>14</output>

<gadget id="calculator">34 - 21</gadget>
<output>13</output>

<gadget id="calculator">14 + 13 + 21</gadget>
<output>48</output>

<result>48</result>