Put 10 identical small balls into three boxes numbered 1, 2, and 3. It is required that the number of balls in each box is not less than its number, so how many different ways are there?
10个相同的小球，放入编号为1，2，3的三个盒子里，要求每个盒子的球数不小于它的编号数，那么不同的放法有多少种．
<gadget id="calculator">5 + 4 + 3 + 2 + 1</gadget>
<output>15</output>

<result>15</result>