N,M=map(int,input().split())
jz=[]
for i in range(N):
    jz.append(input())
sum0=(5-jz[0].count('W'))+(5-jz[1].count('B'))
for i in range(2,N):
    sum0+=5-jz[i].count('R')
for i in range(1,N-1):
    sum1=0
    for j in range(1,i+1):
          sum1+=5-jz[j].count('B')
    for k in range(i+1,N):
          sum1+=5-jz[k].count('R')
    sum1+=5-jz[0].count('W')      
    if sum1<=sum0:
        sum0=sum1
print(sum0)