The 15 balls are divided into four piles with different numbers. How many balls does the pile with the largest number have at least?
15个球分成数量不同的四堆，数量最多的一堆至少有多少个球？
<gadget id="calculator">1 + 2 + 3</gadget>
<output>6</output>

<gadget id="calculator">15 - 6</gadget>
<output>9</output>

<result>9</result>