window_of_col-2     positions(col-2)    sum of col-3 (coverage) based on window
---------------     --------            ---------------
1-20                1,4,7               9
21-40               22,24,38            21 
41-60               44,50,57,60         85
61-80               65                  30

And, I want to print the results as below:
window SUM
20  9
40  21
60  85
80  30