P3392 Painting Stripes

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.

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).

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

### Sample Explanation Target state is: ```plain 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