You are given an $m \times n$ grid, where each cell contains a directional sign indicating which neighboring cell to move to next. The sign in a cell grid[i][j] can be:

• 1: Move right, i.e., from grid[i][j] to grid[i][j + 1].

• 2: Move left, i.e., from grid[i][j] to grid[i][j - 1].

• 3: Move down, i.e., from grid[i][j] to grid[i + 1][j].

• 4: Move up, i.e., from grid[i][j] to grid[i - 1][j].

Note: Some signs may point outside the boundaries of the grid.

Your starting position is the top-left cell $(0, 0)$. A valid path is any sequence of moves beginning at $(0,0)$ and ending at the bottom-right cell $(m - 1, n - 1)$, where each move follows the direction of the sign in the current cell.

You are allowed to change the direction of a sign in any cell, but each modification incurs a cost of 1, and each sign can be modified only once.

Your task is to determine the minimum total cost required to ensure that at least one valid path exists from the top left to the bottom right cell.

Constraints:

• $n ==$ grid.length

• $m==$ grid[i].length

• $1 \leq m, n \leq 50$

• $1 \leq$ grid[i][j] $\leq 4$