NM = [0, 0]
count = 0
for i in input().split():
    if count == 0:
        NM[0] = int(i)
        count += 1
    else:
        NM[1] = int(i)
l = [0 for _ in range(31)]
r = 100000
def pattern(t):
    p1 = list("WBWBWBWB")
    p2 = list("BWBWBWBW")
    r1 = 0
    r2 = 0
    for i in range(2):
        for j in range(0, 7, 2):
            if i == 0:
                for k in range(8):
                    if p1[k] != list(t[j])[k]:
                        r1 += 1
            else:
                for k in range(8):
                    if p2[k] != list(t[j])[k]:
                        r2 += 1
        for j in range(1, 8, 2):
            if i == 0:
                for k in range(8):
                    if p2[k] != list(t[j])[k]:
                        r1 += 1
            else:
                for k in range(8):
                    if p1[k] != list(t[j])[k]:
                        r2 += 1
    if r1 > r2:
        return r2
    else:
        return r1
for i in range(NM[0]):
    l[i] = input()
for i in range(NM[1]-7):
    for j in range(NM[0]-7):
        t = [0 for _ in range(8)]
        for k in range(j, j+8):
            t[k-j] = l[k][i:i+8]
        if pattern(t) < r:
            r = pattern(t)
print(r)