#include<iostream>
using namespace std;
int main()
{
    int n, m; cin >> n >> m;
    int min = 5000;
    char arr[55][55];
    for (int i = 1; i <= n; i++)
    {
        for (int j = 1; j <= m; j++)
        {
            cin >> arr[i][j];
        }
    }
    int num = 0;
    for (int i = 2; i <= n - 1; i++)//B颜色的上界
    {
        for (int j = 1; j < i; j++)
        {
            for (int l = 1; l <= m; l++)
            {
                if (arr[j][l] != 'W')
                {
                    num++;//涂W颜色次数
                }
            }
        }
        for (int j = i; j <= n - 1; j++)//B颜色的下界
        {
            for (int k = i; k <= j; k++)
            {
                for (int l = 1; l <= m; l++)
                {
                    if (arr[k][l] != 'B')
                    {
                        num++;//W和B次数
                    }
                }
            }
            for (int p = j + 1; p <= n; p++)
            {
                for (int q = 1; q <= m; q++)
                {
                    if (arr[p][q] != 'R')
                    {
                        num++;
                    }
                }
            }
            if (num < min)
            {
                min = num;
            }
            num=0;
        }
    }
    cout << min;
    return 0;
}