#include <iostream>

using namespace std;

int N, ans, a, o, maxi;
string s;

int t[1000000][2];
int d[4][4];

int id(char x){
    if(x=='A') return 0;
    if(x=='C') return 1;
    if(x=='T') return 2;
    return 3;
}

int f(int x, int y, int z){
    return min(min(d[x][y],d[y][z]),d[z][x]);
}

int g(int x, int y, int z){
    return max(f(x,y,z),f(x,z,y));
}

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

    cin >> s;
    N = s.length();
    for(int i=0; i<N; i++) t[i][0] = id(s[i]);

    cin >> s;
    for(int i=0; i<N; i++) t[i][1] = id(s[i]);

    for(int i=0; i<N; i++) d[t[i][0]][t[i][1]]++;

    d[0][0] = 0;
    d[1][1] = 0;
    d[2][2] = 0;
    d[3][3] = 0;


    for(int i=0; i<4; i++){
        for(int j=i+1; j<4; j++){
            int h = min(d[i][j],d[j][i]);
            ans += h;
            d[i][j] -= h;
            d[j][i] -= h;
        }
    }

    //cout << ans << endl;

    maxi = 0;

    a = g(0,1,2)+g(0,2,3);
    //cout << a << endl;
    a = min(a,max(d[0][2],d[2][0]));
    maxi = max(maxi,a);

    a = g(0,1,2)+g(0,1,3);
    //cout << a << endl;
    a = min(a,max(d[0][1],d[1][0]));
    maxi = max(maxi,a);

    a = g(0,1,2)+g(1,2,3);
    //cout << a << endl;
    a = min(a,max(d[2][1],d[1][2]));
    maxi = max(maxi,a);

    a = g(0,1,3)+g(1,2,3);
    //cout << a << endl;
    a = min(a,max(d[1][3],d[3][1]));
    maxi = max(maxi,a);

    a = g(0,1,3)+g(0,2,3);
    //cout << a << endl;
    a = min(a,max(d[0][3],d[3][0]));
    maxi = max(maxi,a);

    a = g(1,2,3)+g(0,2,3);
    //cout << a << endl;
    a = min(a,max(d[3][2],d[2][3]));
    maxi = max(maxi,a);

    //cout << maxi << endl;

    for(int i=0; i<4; i++){
        for(int j=0; j<4; j++){
            if(i!=j) o += d[i][j];
        }
    }

    ans += (maxi*2);

    o -= (maxi*3);

    o /= 4;

    ans += (o*3);

    cout << ans << endl;

    return 0;
}