n, m = map(int, input().split())
blue = [0] * n  # 记录各颜色每一行的个数
white = [0] * n
red = [0] * n
for i in range(n):
    s = input().strip()
    for j in s:
        if j == 'R':
            red[i] += 1
        elif j == 'B':
            blue[i] += 1
        elif j=='W':
            white[i] += 1
girds = 0
min = float('inf')
for l in range(1, n - 1):  # blue的宽度
    for j in range(1, n - l):  # blue的起点
        for k in range(j, j + l):  # 涂成蓝色
            girds += m - blue[k]
        for q in range(j):  # 涂成白色
            girds += m - white[q]
        for p in range(j + l, n):  # 涂成红色
            girds += m - red[p]
        print(girds)
        if girds < min:
            min = girds
            girds = 0
print(min)
#主要思路是控制中间蓝色区块的宽度与起点，然后根据白蓝红的顺序进行累加