n, m = map(int, input().split())
s = list(input())
t = list(input())
top_a = 0
top_b = 0
botom_a = 0
botom_b = 0
for i in s:
    if i == "A":
        top_a += 1
    elif i == "B":
        top_b += 1
for i in t:
    if i == "A":
        botom_a += 1
    elif i == "B":
        botom_b += 1
if top_a > botom_a:
    a = botom_a
else:
    a = top_a
if top_b > botom_b:
    b = botom_b
else:
    b = top_b
print(a + b)