n,m=map(int,input().split())
s=list(input())
t=list(input())
from collections import Counter,defaultdict
sd=defaultdict(int)
td=defaultdict(int)
for i in s:
    sd[i]+=1
for j in t:
    td[j]+=1
ans=min(sd['A'],td['A'])+min(sd['B'],td['B'])
print(ans)