Discrete Mathematics
# Grid Walking

A dog is standing at the bottom left corner of a grid of $46 \times 46$ streets. The dog is trying to get to the top right corner of the grid, where it knows there is some food. As the dog runs, between the corners, it will only ever run up and to the right. Any time the dog runs to the right, it runs at least $4$ consecutive blocks to the right, and any time it runs up, it runs at least $12$ consecutive blocks up. How many different intersections are unreachable for the dog by following these rules?

**Details and assumptions**

The last stretch that the dog runs must also satisfy the condition on the minimum number of consecutive blocks.

An intersection is **reachable** if the dog runs through it. It doesn't matter if the dog can change direction at that intersection. Remember that the dog needs to exit through the top right corner of the grid.

$10\times 10$ grid to the top right corner in 20 steps, if you must cross the main diagonal (in red) two times or more?

How many ways are there to get from the bottom left corner of aIn the above example, the blue path crosses the red diagonal three times, so it is a valid path.

The paths are monotonic, i.e, only moves to the right and the top are allowed.

Start in the lower left corner of a 5x10 unit grid. You may only move to the right and upward along the edges to get to the top right corner. The ** area** of a path between the corners is the amount of area beneath the line in square units. For example:

**On a 5x10 grid, how many different paths from the lower left corner to the upper right corner have an area congruent to 1 mod 5?**

Bonus: Choose any prime $p$ and positive integers $i<p$, $j$ and $k$ and generalize this solution to the number of paths with an area congruent to $i$ mod $p$ on a $jp$ by $kp$ grid.