Question 150
Rows and columns of a $29 \times 29$ grid are numerated from bottom to top and from left to right, respectively by numbers $1, 2, \ldots, 29$. Some unit squares of the grid are marked. For each marked unit square there is at most one other marked unit square whose row number is not less than the row number and column number is not less than the column number of the initial unit square. Find the largest possible number of marked unit squares on the grid.