Since my solution is too messy,I don’t think you’ll be able to understand how I did it just by looking at my code.
So Here’s a simple explantion by an example:
consider A as 5 and B as 243.

My Solution:
I broke 5…243 into three parts-

5…99 100…199 200…243

first part(5…99)-
5…9 10…19 20…29 30…39 and so on upto 90…99

lets take 10…19
1 is repeated 10 times and sum of 0-9 is precomputed and stored
20…29
2 is repeated 10 times and sum of 0-9 is precomputed
and so on…

second part(100-199)-
1 is repeatedd 100 times and sum 0-99 is precomputed.

third part(200-243)-
2 is repeated 44 times and solve for 43 using the same recursion strategy.