1. In one move, you can either add one of its digits to the number or subtract one of its digits from the number (for example, from the number 142 you can get $142+2=144, 142-4=138$ and several other numbers).

a) Can you get the number 2021 from the number 2020 in several moves?

b) Can you get the number 2021 from the number 1000 in several moves?
Solution. a) Yes, for example, like this: $20 \mathbf{2 0} \rightarrow 20 \mathbf{1 8} \rightarrow \mathbf{2 0 1 9} \rightarrow 2021$.

b) Yes. For example, by adding the first digit (one), we can reach the number 2000; by adding the first digit (two), we can reach 2020; then see part a.

Criteria. Part a) 3 points, b) 4 points. In part (b), 1 point is given for progressing to 1999 (for example, for a "solution" of the form "from 1000, we add one at a time and there is the result 2021").