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

s_a_num = S.count("A")
t_a_num = T.count("A")

s_b_num = S.count("B")
t_b_num = T.count("B")

ans = min(s_a_num,t_a_num) + min(s_b_num, t_b_num)

print(ans)