Answer:
4347
Explanation:
Let me have a bold guess...
The largest sum of 2 numbers chosen is 2020+2021 = 4041
The smallest sum of 2 numbers chosen is -153 + -152 = -305
Then all the possible sum must drop in between those 2 numbers
-305, -304, -303 ......,-1, 0 , 1,.... 4039, 4040, 4041:
↓ ↓ ↓
305 + 1 + 4041 = 4347
I wish this is correct ......