A certain bus has 14 stations including the start point and the end point. There is a bus except the end point. Among the passengers who get on the bus at each stop, exactly one passenger gets off at each subsequent stop. In order to make each passenger have Seats, how many seats does this bus have at least?
某路公共汽车，包括起点和终点共有14个车站，有一辆车除终点外，每一站上车的乘客中，恰好有一位乘客到以后的每一站下车，为了使每位乘客都有座位，问这辆公共汽车最少要有多少个座位？
<gadget id="calculator">13 - 0</gadget>
<output>13</output>

<gadget id="calculator">12 - 1</gadget>
<output>11</output>

<gadget id="calculator">11 - 2</gadget>
<output>9</output>

<gadget id="calculator">10 - 3</gadget>
<output>7</output>

<gadget id="calculator">9 - 4</gadget>
<output>5</output>

<gadget id="calculator">8 - 5</gadget>
<output>3</output>

<gadget id="calculator">7 - 6</gadget>
<output>1</output>

<gadget id="calculator">13 + 11 + 9 + 7 + 5 + 3 + 1</gadget>
<output>49</output>

<result>49</result>