A series of numbers 1, 5, 9, 13, 17, ... starting from the second item, the difference between the subsequent item minus its previous item is equal, counting from left to right, what number is 197.
一列数1，5，9，13，17，…从第二项起，后项减去它的前一项的差都相等，从左向右数起，第多少个数是197．
<gadget id="calculator">197 - 1</gadget>
<output>196</output>

<gadget id="calculator">5 - 1</gadget>
<output>4</output>

<gadget id="calculator">196 / 4</gadget>
<output>49</output>

<gadget id="calculator">49 + 1</gadget>
<output>50</output>

<result>50</result>