from sys import stdin, stdout
input = lambda: stdin.readline().rstrip()
write = stdout.write

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

    a = min(S.count('A'), T.count('A'))
    b = min(S.count('B'), T.count('B'))
    print(a + b)

main()