#include <iostream>
using namespace std;
int main()
{
    int n = 0;
    int m = 0;
    char a[1000] [1000] = {0};
    int s = 0;
    int min = 100000000;
    cin>>n>>m;
    for (int i = 0;i<n;i++)
        for (int j = 0;j<m;j++)
            cin>>a[i][j];
    for (int i = 0;i<n;i++)
        for (int j = 0;j<m;j++)
        {
            for (int y = 0;y<i;y++)
                for (int z = 0;z<m;z++)
                    if (a[y][z] != 'W')
                        s++;
            for (int y = 0;y<j;y++)
                for (int z = 0;z<m;z++)
                    if (a[y][z] != 'B')
                        s++;
            for (int y = 0;y<n;y++)
                for (int z = 0;z<m;z++)
                    if (a[y][z] != 'R')
                        s++;
            if (s < min)
                min = s;
        }
    cout<<min<<endl;
}