#include <bits/stdc++.h>
using namespace std;

#define LL long long
#define LD long double
#define PII pair<int, int>
#define VI vector<int>
#define VPII vector<PII>
#define f first
#define s second
#define MP make_pair
#define PB push_back
#define endl '\n'
#define SIZ(c) (int)(c).size()
#define ALL(c) (c).begin(), (c).end()
#define REP(i, n) for (int i = 0; i < (int)(n); i++)
#define FOR(i, b, e) for (int i = (b); i <= (int)(e); i++)
#define FORD(i, b, e) for (int i = (b); i >= (int)(e); i--)
#define ll LL
#define st f
#define nd s
#define mp MP
#define pb PB
#define eb emplace_back
#define siz(c) SIZ(c)
const int inf = 1e9 + 7;
const LL INF = 1e18L + 7;

#define sim template<class n
sim, class s> ostream & operator << (ostream &p, pair<n, s> x)
{return p << "<" << x.f << ", " << x.s << ">";}
sim> auto operator << (ostream &p, n y) ->
typename enable_if<!is_same<n, string>::value, decltype(y.begin(), p)>::type
{int o = 0; p << "{"; for (auto c : y) {if (o++) p << ", "; p << c;} return p << "}";}
void dor() {cerr << "\n";}
sim, class...s> void dor(n p, s...y) {cerr << p << " "; dor(y...);}
sim, class s> void mini(n &p, s y) {if (p > y) p = y;}
sim, class s> void maxi(n &p, s y) {if (p < y) p = y;}

#ifdef DEB
#define debug(...) dor(__FUNCTION__, ":", __LINE__, ": ", __VA_ARGS__)
#else
#define debug(...)
#endif

#define I(x) #x " =", (x), " "

string s, t;
map<char, int> mapa;

int ile[5][5], n;

int32_t main()
{
	mapa['A'] = 0;
	mapa['C'] = 1;
	mapa['G'] = 2;
	mapa['T'] = 3;
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	cin >> s >> t;
	n = SIZ(s);
	int all = 0, res = 0;
	FOR(i, 0, n - 1)
	{
		if (mapa[s[i]] != mapa[t[i]])
		{
			ile[mapa[s[i]]][mapa[t[i]]]++;
			all++;
		}
	}
	FOR(i, 0, 3)
	{
		FOR(j, i + 1, 3)
		{
			int x = min(ile[i][j], ile[j][i]);
			ile[i][j] -= x;
			ile[j][i] -= x;
			res += x;
			all -= 2 * x;
		}
	}
	VI X{0, 1, 2, 3};
	do
	{
		int x = min(ile[X[0]][X[1]], min(ile[X[1]][X[2]], ile[X[2]][X[0]]));
		ile[X[0]][X[1]] -= x;
		ile[X[1]][X[2]] -= x;
		ile[X[2]][X[0]] -= x;
		res += 2 * x;
		all -= 3 * x;
	} while (next_permutation(ALL(X)));
	assert(all % 4 == 0);
	res += all / 4 * 3;
	cout << res << "\n";
}