#ifndef LOCAL
#pragma GCC optimize("O3")
#endif
#include <bits/stdc++.h>
using namespace std;
#define sim template <class c
#define mor > muu & operator<<(
#define ris return *this
#define R22(r) sim> typename enable_if<1 r sizeof(dud<c>(0)),muu&>::type operator<<(c g) {
sim> struct rge {c b, e;};
sim> rge<c> range(c i, c j) {return rge<c>{i, j};}
sim> auto dud(c*r)->decltype(cerr << *r);
sim> char dud(...);
struct muu {
	#ifdef LOCAL
	stringstream a;
	~muu() {cerr << a.str() << endl;}
	R22(<) a << boolalpha << g; ris;}
	R22(==) ris << range(begin(g), end(g));}
	sim mor rge<c> u) {
		a << "[";
		for (c i = u.b; i != u.e; ++i) 
			*this << ", " + 2 * (i == u.b) << *i;
		ris << "]";
	}
	sim, class b mor pair <b,c> r){ris << "(" << r.first << ", " << r.second << ")";}
	#else
	sim mor const c&){ris;}
	#endif
};
#define debug muu() << __FUNCTION__ << "#" << __LINE__ << ": "
#define imie(r...) "[" #r ": " << (r) << "] "
#define range(a, b) "[[" #a ", " #b "): " << range(a, b) << "] "
#define arr(a, i) "[" #a imie(i) ": " << a[i] << "] "
using ll = long long;
using ld = long double;
using pii = pair <int, int>;
int maska[4][4];
int kol['Z' + 7];
int main() {
	kol['A'] = 0;
	kol['C'] = 1;
	kol['G'] = 2;
	kol['T'] = 3;
	string s1, s2;
	cin >> s1 >> s2;
	int N = s1.size();
	int kr = 0;
	for (int i = 0; i < N; i++) {
		if (s1[i] != s2[i]) {
			maska[kol[s1[i]]][kol[s2[i]]]++;
			kr++;
		}
	}
	int cykle = 0;
	for (int i = 0; i < 4; i++) {
		for (int w = 0; w < 4; w++) {
			if (i != w) {
				int g = min(maska[i][w], maska[w][i]);
				maska[i][w] -= g;
				maska[w][i] -= g;
				cykle += g;
			}
		}
	}
	for (int i = 0; i < 4; i++) {
		for (int w = 0; w < 4; w++) {
			for (int k = 0; k < 4; k++) {
				if (i != w && w != k) {
					int g = min(maska[i][w], maska[w][k]);
					g = min(g, maska[k][i]);
					maska[i][w] -= g;
					maska[w][k] -= g;
					maska[k][i] -= g;
					cykle += g;
				}
			}
		}
	}
	int maxi = 0;
	for (int i = 0; i < 4; i++) {
		for (int w = 0;w < 4; w++) {
			maxi = max(maxi, maska[i][w]);
		}
	}
	cykle += maxi;
	printf("%d", kr - cykle);
}