Count of Letters

Problem Background

Special for beginners, ^_^

Problem Description

You are given an uppercase letter matrix of size $R \times S$. Starting from the top-left corner, you may move up, down, left, or right, and you may not step onto a letter you have already visited. What is the maximum number of distinct letters you can visit?

Input Format

The first line contains the number of rows $R$ and the number of columns $S$ of the letter matrix, with $1\le R,S\le 20$.

Then $R$ lines follow, each containing a row of the $S$-column letter matrix.

Output Format

The maximum number of distinct letters that can be visited.

Sample Input

3 6
HFDFFB
AJHGDH
DGAGEH

Sample Output

6

Constraints

1s, 1024KiB for each test case.