str1 = input()
str2 = input()

import collections
import itertools

pairs = collections.Counter()

for i in range(len(str1)):
    pairs[str1[i] + str2[i]] += 1

switches = 0

test2 = [c+d for c in "ACGT" for d in "ACGT" if c != d]
test3 = [x[:-1] for x in itertools.permutations("ACGT")]
test4 = list(itertools.permutations("ACGT"))

test = test2 + test3 + test4

for t in test:
    m = min(pairs[t[i] + t[(i+1)%len(t)]] for i in range(len(t)))
    switches += m * (len(t)-1)
    for i in range(len(t)):
        pairs[t[i] + t[(i+1)%len(t)]] -= m

print(switches)