If the first diagonal goes through (1,8), the second diagonal goes through (7,1) and (2,8) etc. What is the pattern to figure out the diagonals?

For example, the point (4, 1) on a 4x4 board goes through the 7th diagonal.
(1, 1) on a 16x16 goes through diagonal 16.
(6, 4) on a 8x8 goes through diagonal 9.