Description

As long as a flag composed of $N \times M$ small squares meets the following rules, it is a valid pattern.

• All cells in the top several rows (at least one row) are white.
• The next several rows (at least one row) are blue.
• The remaining rows (at least one row) are red.

There is a cloth divided into $N$ rows and $M$ columns of cells, where each cell is one of white, blue, or red. Xiao A hopes to turn this cloth into a valid pattern by painting some cells to cover their previous colors.

Xiao A is lazy and wants to repaint the fewest cells so that the cloth becomes a valid pattern.

Input Format

The first line contains two integers $N, M$.

Each of the next $N$ lines is a string of length $M$, where each character is one of W (white), B (blue), or R (red).

Output Format

A single integer, the minimum number of cells that need to be repainted.

4 5
WRWRW
BWRWB
WRWRW
RWBWR

11

Hint

Sample Explanation

Target state is:

WWWWW
BBBBB
RRRRR
RRRRR

In total, $11$ cells need to be changed.

Constraints

For $100\%$ of the testdata, $N, M \leq 50$.

Translated by ChatGPT 5