9. In the number table below, both the upper and lower rows of numbers are arithmetic sequences. For the corresponding numbers in the upper and lower rows, the difference between the larger number and the smaller number, the smallest is $\qquad$ _
【Analysis】The first sequence is $5 a(a=1,2,3, \ldots)$; the second sequence is $2020-7 a \quad(a=1,2,3, \ldots)$ they approach 12 each time, the initial difference is $2013-5=2008, \quad 2008 \div 12=168 \ldots \ldots .4$, so the smallest difference is 4 ( above is 840, below is 844 ).