Let $N$ be a positive integer. Consider a $N \times N$ array of square unit cells. Two corner cells that lie on the same longest diagonal are colored black, and the rest of the array is white. A [i]move[/i] consists of choosing a row or a column and changing the color of every cell in the chosen row or column. 
What is the minimal number of additional cells that one has to color black such that, after a finite number of moves, a completely black board can be reached?
1. **Base Case:**
   - For \( n = 2 \), the \( 2 \times 2 \) grid has two black cells on the diagonal. We can turn the entire grid black with 2 moves (one for each row or column). No additional cells need to be colored black. Thus, the minimal number of additional cells is \( 0 \).

2. **Inductive Hypothesis:**
   - Assume that for an \( n \times n \) grid, the minimal number of additional cells that need to be colored black is \( 2n - 4 \).

3. **Inductive Step:**
   - Consider an \((n+1) \times (n+1)\) grid. We need to show that the minimal number of additional cells that need to be colored black is \( 2(n+1) - 4 = 2n - 2 \).
   - By the inductive hypothesis, for the \( n \times n \) grid, we need \( 2n - 4 \) additional black cells.
   - When we move to an \((n+1) \times (n+1)\) grid, we add one row and one column. To ensure that the entire grid can be turned black, we need to color additional cells in the new row and column.
   - Specifically, we need to color the cells in the second to the \( n \)-th positions of the first row and the second to the \( n \)-th positions of the last column. This adds \( 2 \) more cells to the \( 2n - 4 \) cells already required for the \( n \times n \) grid.
   - Therefore, the total number of additional cells required is \( 2n - 4 + 2 = 2n - 2 \).

4. **Construction:**
   - On an \( n \times n \) grid, color the cells from the second to the \((n-1)\)-th positions in the first row and the second to the \((n-1)\)-th positions in the last column black. This ensures that after a finite number of moves, the entire grid can be turned black.

5. **Conclusion:**
   - By induction, the minimal number of additional cells that need to be colored black for an \( n \times n \) grid to be turned completely black after a finite number of moves is \( 2n - 4 \).

The final answer is \( \boxed{ 2n - 4 } \).