n, m = map(int, input().split())
up = input()
down = input()
up_a = up.count('A')
up_b = n - up_a
down_a = down.count('A')
down_b = m - down_a
if up_a > down_a:
    wear_a = down_a
else:
    wear_a = up_a
if up_b > down_b:
    wear_b = down_b
else:
    wear_b = up_b
print(wear_a + wear_b)