Find the sum of Matrix[i][j] whose values are given by abs(i-j)

Given a matrix of size NxN. Rows and Columns are numbered from 0 to N-1. jth column of ith row contains absolute difference between i and j.
In other words, Matrix[i][j] = abs(i-j) where 0 ≤ i, j < N.

you have to find sum of this matrix 
i.e.,
sum = 0
for i=0 to N-1
    for j=0 to N-1
        sum += Matrix[i][j]



For example:

If the size of the matrix is 2
Matrix will be:
0 1
1 0
Sum = 0 + 1 + 1 + 0
        = 2

If the size of the matrix is 3
Matrix will be:
0 1 2
1 0 1
2 1 0
Sum = 0 + 1 + 2 + 1 + 0 + 1 + 2 + 1 + 0
        = 8


Implementation: