input()
top_plain = 0
under_plain = 0
top_shank = 0
under_shank = 0
for s in input():
    if s == "A":
        top_plain+=1
    else:
        top_shank+=1
for s in input():
    if s == "A":
        under_plain+=1
    else:
        under_shank+=1
days = min(top_plain,under_plain) + min(top_shank,under_shank)
print(days)