_ = input()
s = input()
t = input()
 
a1 = 0
a2 = 0
b1 = 0
b2 = 0
for c in s:
    if c == 'A':
        a1 += 1
    else:
        b1 += 1
for c in t:
    if c == 'A':
        a2 += 1
    else:
        b2 += 1
 
ans = min(a1, a2) + min(b1, b2)
print(ans)