import itertools

s1 = input()
s2 = input()
n = len(s1)

dns = "ACGT";
m = {str(i)+str(j) : 0  for i in dns for j in dns}

for i in range(n):
	if s1[i] != s2[i]:
		m[s1[i]+s2[i]] += 1

#print(m)

ans = 0

for i in range(4):
	for j in range(i+1, 4):
		prs = min(m[dns[i]+dns[j]], m[dns[j]+dns[i]])
		ans += prs
		m[dns[i]+dns[j]] -= prs
		m[dns[j]+dns[i]] -= prs

#print(ans, m)

for ss in itertools.permutations(dns, 3):
	s = ''.join(ss)
	#print(s)
	hrmas = min(m[s[0:2]], m[s[1:3]], m[s[2]+s[0]])
	#if hrmas > 0:
	#	print(s, hrmas)
	ans += 2*hrmas
	m[s[0:2]] -= hrmas
	m[s[1:3]] -= hrmas
	m[s[2]+s[0]] -= hrmas

#print(ans, m)	

ans += 3 * max(m.values())

print(ans)