谁能为我提供二维二进制索引树的算法？

I searched on internet but couldn't find a good one. I got some help from geeksforgeeks.org but can't understand the construction part where we are subtracting v1-v2-v2-v4+v3 from aux[i][j] while updating the BIT array. Just let me know why we are subtracting here.I searched on internet but couldn't find a good