N,M=list(map(int,input().split()))
S=list(input())
T=list(input())

dic_a={}
dic_b={}

for s in ["A","B"]:
    dic_a[s]=0
    dic_b[s]=0

for s in S:
    dic_a[s]+=1
for t in T:
    dic_b[t]+=1
    
print(min(dic_a["A"],dic_b["A"])+min(dic_a["B"],dic_b["B"]))