Sum of array's elements in the lower right triangular part

The function takes as input at the most 2-D array. It computes the sum of the elements that are in the lower right triangular part

The function takes as input at the most two-dimensional array. It computes the sum of the elements of that are in the lower right triangular part(counter-diagonal elements).
For example, if the input is [1 2; 3 4; 5 6; 7 8], then the function would return 21.