#include<iostream>
using namespace std;

char a[100][100];
int n, m, i, j, k, x, temp, Min = 0;
int main()
{
	cin >> n >> m;
	for (i = 1; i <= n; i++)
	{
		for (j = 1; j <= m; j++)
		{
			cin >> a[i][j];
		}
	}
	for ( i = 1; i <= n-2; i++)
	{
		temp = 0;
		for( x = 1; x <= m; x++)
			if (a[i][x] != 'W')
				temp++;
		for ( j = i+1; j <= n-1; j++)
		{
			for (x = 1; x <= m; x++)
				if (a[j][x] != 'B')
					temp++;
			for (k = j + 1; k <= n; k++)
			{
				for (x = 1; x <= m; x++)
					if (a[k][x] != 'R')
						temp++;
				if (i == 1 && j == i + 1 && k == n)
					Min = temp ;
				if (temp < Min)
					Min = temp ;
			}
		}
	}
	cout << Min << endl;
	return 0;
}
                       ```