Task

Find and display the sum of elements that are below the main diagonal of a matrix.

The matrix should be a square matrix.



 ───   Matrix to be used:   ───


     [[1,3,7,8,10],
      [2,4,16,14,4],
      [3,1,9,18,11],
      [12,14,17,18,20],
      [7,1,3,9,5]] 



m = {{1, 3, 7, 8, 10}, {2, 4, 16, 14, 4}, {3, 1, 9, 18, 11}, {12, 14, 17, 18, 20}, {7, 1, 3, 9, 5}};
Total[LowerTriangularize[m, -1], 2]