#include <bits/stdc++.h>

using namespace std;

int main() {
    ios_base::sync_with_stdio(0);

    string a, s;
    cin >> a >> s;

    vector<int> M(150, 0);
    M['A'] = 0;
    M['C'] = 1;
    M['G'] = 2;
    M['T'] = 3;

    int n = a.size();
    vector<vector<int>> F(4, vector<int>(4, 0));
    for (int i = 0; i < n; i++) {
        F[M[a[i]]][M[s[i]]]++;
    }

    int res = 0;
    for (int i = 0; i < 4; i++) {
    for (int j = 0; j < 4; j++) {
        if (i == j)
            continue;
        int d = min(F[i][j], F[j][i]);
        res += d;
        F[i][j] -= d;
        F[j][i] -= d;
    }}

    for (int i = 0; i < 4; i++) {
    for (int j = 0; j < 4; j++) {
    for (int k = 0; k < 4; k++) {
            if (i == j  ||  j == k || k == i)
                continue;
            int d = min(F[i][j], min(F[j][k], F[k][i]));
            res += 2 * d;
            F[i][j] -= d;
            F[j][k] -= d;
            F[k][i] -= d;
    }}}

    for (int i = 1; i < 4; i++)
        res += 3 * F[0][i];
    cout << res << endl;

    return 0;
}