Unique Paths - The Formula

To reach cell $(i, j)$, you came from either $(i-1, j)$ (from above) or $(i, j-1)$ (from left).

The number of paths to $(i, j)$ is the sum of paths to those two cells: $dp[i][j] = dp[i-1][j] + dp[i][j-1]$.

Base case: The top row and left column each have exactly 1 path (you can only move in one direction).