Observe the arrangement of the five numbers on the right: 19, 37, 55, a, 91, and infer how much a =.
观察右面的五个数：19、37、55、a、91排列的规律，推知a=多少．
<gadget id="calculator">55 + 18</gadget>
<output>73</output>

<result>73</result>