22. Given 5 numbers: $a_{1}=-1, a_{2}=0, a_{3}=1, a_{4}=2, a_{5}=3$. The sixth is the product of the first and the second, the seventh is the product of the second and the third, the eighth is the product of the third and the fourth, and so on. What is the last non-zero number in this sequence?
22. Let's construct a table:

| $a_{1}$ | $a_{2}$ | $a_{3}$ | $a_{4}$ | $a_{5}$ | $a_{6}$ | $a_{7}$ | $a_{8}$ | $a_{9}$ | $a_{10}$ | $a_{11}$ | $a_{12}$ | $a_{13}$ | $a_{14}$ | $a_{15}$ | $a_{16}$ | $a_{17}$ |
| :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: |
| -1 | 0 | 1 | 2 | 3 | 0 | 0 | 2 | 6 | 0 | 0 | 0 | 12 | 0 | 0 | 0 | 0 |

Starting from $a_{14}$, all numbers will be zeros, so the last non-zero number will be $a_{13}=12$.