N,M = list(map(int,input().split()))
S = input()
T = input()
ueA = 0
ueB = 0
for i in S:
    if i == 'A':ueA += 1
    else:ueB += 1
sitaA = 0
sitaB = 0
for i in T:
    if i == 'A':sitaA += 1
    else:sitaB += 1
print(min(ueA,sitaA) + min(ueB,sitaB))