#include <bits/stdc++.h>

using namespace std;

typedef long double ld;
typedef long long ll;
typedef pair<int,int> ii;
typedef pair<double,double> dd;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<vector<int>> vvi;
typedef vector<ii> vii;

map<char, int> M;

int main(){
    M['A'] = 0;
    M['C'] = 1;
    M['G'] = 2;
    M['T'] = 3;
    vvi T(4, vi(4, 0));

    ios::sync_with_stdio(false);
    string s1, s2; cin >> s1 >> s2;
    int N  = s1.length();

    for(int i = 0; i < N; ++i) T[M[s1[i]]][M[s2[i]]]++;

    int res = 0;
    int z = 0;
    while(true) {
        // for(int i = 0; i < 4; ++i) {
        //     for(int j = 0; j < 4; ++j) {
        //         cout << T[i][j] << " ";
        //     }
        //     cout << endl;
        // }
        // cout << endl;
        for(int i = 0; i < 4; ++i) T[i][i] = 0;
        for(int i = 0; i < 4; ++i) for (int j = i+1; j < 4; ++j) {
            int x = min(T[i][j], T[j][i]);
            res += x;
            T[i][j] -=x;
            T[j][i] -= x;
        }



        for(int i = 0; i < 4; ++i) {
            int nonzero = 0;
            for(int j = 0; j < 4; ++j) if (T[i][j] > 0) nonzero++;
            if (nonzero == 1) {
                int k;
                for(int j = 0; j < 4; ++j) if (T[i][j] > 0) k = j;
                for(int j = 0; j < 4; ++j) {
                    int s = min(T[i][k], T[k][j]);
                    T[i][k] -= s;
                    res += s;
                    T[i][j] += s;
                    T[k][j] -= s;
                }
                break;
            }
        }

        int s = 0;
        for(int i = 0; i < 4; ++i) for(int j = 0; j < 4; ++j) s += T[i][j];
        if (s == 0) break;
    }

    cout << res << endl;
}