How many different sums can be obtained by writing 15 consecutive natural numbers in a line and adding up every 4 adjacent numbers?
用连续的15个自然数写成一行，每相邻的4个数相加，可得到多少种不同的和．
<gadget id="calculator">15 - 4</gadget>
<output>11</output>

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

<result>12</result>