Description

Given a matrix of uppercase letters with dimensions $row \times col$, starting from the top-left corner, you can move in four directions: up, down, left, or right. However, you cannot move to a cell containing a letter that has already been visited. Determine the maximum number of distinct letters that can be traversed.

Input Format

The first line contains the number of rows $R$ and columns $S$ of the letter matrix, where $1 \leq R, S \leq 20$. The following $R$ lines each contain $S$ uppercase letters representing the matrix.

Output Format

The maximum number of distinct letters that can be traversed.

3 6
HFDFFB
AJHGDH
DGAGEH

6