#include <bits/stdc++.h>

using namespace std;

typedef long long ll;

#define rep(i, n) for (int i = 0; i < (n); i++)

int f(char c) {
	if (c == 'A') return 0;
	if (c == 'C') return 1;
	if (c == 'G') return 2;
	if (c == 'T') return 3;
}

int main(void){
	string s1, s2; cin >> s1 >> s2;
	vector<vector<int>> t(4, vector<int>(4));
	for (int i = 0; i < s1.size(); i++) t[f(s1[i])][f(s2[i])]++;
	int ans = 0;
	for (int i = 0; i < 4; i++) for (int j = 0; j < i; j++) {
		int x = min(t[i][j], t[j][i]);
		t[i][j] -= x;
		t[j][i] -= x;
		ans += x;
	}
	for (vector<int> c : vector<vector<int>>{{0,1,2}, {0,1,3}, {0,2,3}, {1,2,3}}) do {
		int x = 1e9;
		for (int i = 0; i < 2; i++) x = min(x, t[c[i]][c[i+1]]);
		x = min(x, t[c[2]][c[0]]);
		ans += 2*x;
		for (int i = 0; i < 2; i++) t[c[i]][c[i+1]] -= x;
		t[c[2]][c[0]] -= x;
	} while (next_permutation(c.begin(), c.end()));
	vector<int> c{0,1,2,3};
	do {
		int x = 1e9;
		for (int i = 0; i < 3; i++) x = min(x, t[c[i]][c[i+1]]);
		x = min(x, t[c[3]][c[0]]);
		ans += 3*x;
		for (int i = 0; i < 3; i++) t[c[i]][c[i+1]] -= x;
		t[c[3]][c[0]] -= x;
	} while (next_permutation(c.begin(), c.end()));
	cout << ans << endl;
}