the value of q ie x,j-1 will store the sum till x,j-1 starting from 0th index which will even contain value of p so wont it be subtracting the same value twice as the sum will be in rowwise order right
Sum from 0,0 index to x,j-1 index in prefix sum approach for 2d matrix
@krishnakumarkrish hi in order to calculate that submatrix sum we have to remove P and Q as this extra part ,only R portion come two times therefore we add one time in final result in order to get result ,the V portion give sum from 0,0 to x,y and P and Q are extra so just remove them and the part which has been removed two times just add once again to final result ,hope you get it 
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