#include <stdio.h> #include <string.h> #include <limits.h>

int min(int a, int b) { return a < b ? a : b; }

int main() { int n, m; scanf("%d %d", &n, &m); char grid[45][55]; for (int i = 0; i < n; i++) { scanf("%s", grid[i]); }

int costW[45], costB[45], costR[45];
for (int i = 0; i < n; i++) {
    int w = 0, b = 0, r = 0;
    for (int j = 0; j < m; j++) {
        if (grid[i][j] == 'W') w++;
        else if (grid[i][j] == 'B') b++;
        else r++;
    }
    costW[i] = m - w;
    costB[i] = m - b;
    costR[i] = m - r;
}

int dp[45][3];
dp[0][0] = costW[0];
dp[0][1] = INT_MAX;
dp[0][2] = INT_MAX;

for (int i = 1; i < n; i++) {
    dp[i][0] = dp[i-1][0] + costW[i];
    if (dp[i-1][0] < INT_MAX && dp[i-1][1] < INT_MAX)
        dp[i][1] = min(dp[i-1][0], dp[i-1][1]) + costB[i];
    else if (dp[i-1][0] < INT_MAX)
        dp[i][1] = dp[i-1][0] + costB[i];
    else
        dp[i][1] = INT_MAX;
        
    if (dp[i-1][1] < INT_MAX && dp[i-1][2] < INT_MAX)
        dp[i][2] = min(dp[i-1][1], dp[i-1][2]) + costR[i];
    else if (dp[i-1][1] < INT_MAX)
        dp[i][2] = dp[i-1][1] + costR[i];
    else
        dp[i][2] = INT_MAX;
}

printf("%d\n", dp[n-1][2]);
return 0;

} INT_MAX换成足够大的数也是可以的