Let $n,k$ be positive integers such that $n>k$. There is a square-shaped plot of land, which is divided into $n\times n$ grid so that each cell has the same size. The land needs to be plowed by $k$ tractors; each tractor will begin on the lower-left corner cell and keep moving to the cell sharing a common side until it reaches the upper-right corner cell. In addition, each tractor can only move in two directions: up and right. Determine the minimum possible number of unplowed cells.
To determine the minimum possible number of unplowed cells, we need to analyze the movement of the tractors on the \( n \times n \) grid. Each tractor starts at the lower-left corner and can only move up or right until it reaches the upper-right corner. 

1. **Invariant Analysis**:
   Each tractor must make exactly \( n-1 \) right moves and \( n-1 \) up moves to reach the upper-right corner. This is because the total number of moves required to go from the lower-left corner to the upper-right corner is \( 2(n-1) \).

2. **Trajectory and Coverage**:
   Each tractor will follow a unique path from the lower-left to the upper-right corner. The goal is to maximize the number of cells covered by these paths. 

3. **Algorithm for Tractor Movement**:
   - The first tractor can take a path that alternates between moving up and right.
   - Subsequent tractors can follow paths that are slightly shifted to ensure maximum coverage of the grid.

4. **Calculation of Covered Cells**:
   - The first tractor covers \( 2n-1 \) cells.
   - Each subsequent tractor will cover new cells until it overlaps with the paths of previous tractors.

5. **Minimizing Unplowed Cells**:
   - To minimize the number of unplowed cells, we need to maximize the number of unique cells covered by the \( k \) tractors.
   - The optimal strategy is to spread the tractors' paths as evenly as possible across the grid.

6. **Mathematical Formulation**:
   - The total number of cells in the grid is \( n^2 \).
   - The maximum number of unique cells that can be covered by \( k \) tractors is \( n^2 - (n-k)^2 \).

7. **Derivation**:
   - If \( k \) tractors are used, the minimum number of unplowed cells is given by the cells that are not covered by any tractor.
   - This can be calculated as the difference between the total number of cells and the maximum number of unique cells covered by the tractors.

   \[
   \text{Minimum unplowed cells} = (n-k)^2
   \]

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