#include <bits/stdc++.h>

using namespace std;

string a,b;

vector<int> arr[4][4];

int visited[1000006];

int counter = 0;
int mySize = 0;

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


bool finished(){
    for(int i=0;i<4;i++){
        for(int j=0;j<4;j++){
            if(i!=j && arr[i][j].size() != 0){
                return false;
            }
        }
    }
    return true;
}

int removePerfect(){
    for(int i=0;i<4;i++){
        for(int j=0;j<4;j++){
            if(i!=j){
                if(!arr[i][j].empty() && !arr[j][i].empty()){
                    visited[arr[i][j].back()] = 1;
                    visited[arr[j][i].back()] = 1;
                    arr[i][j].pop_back();
                    arr[j][i].pop_back();
                    return 1;
                }
            }
        }
    }
    return 0;
}

int removeNonPerfect(){
    for(;counter>=0;counter--){
        if(!visited[counter]){
            visited[counter] = 1;
            int druhy = help(b[counter]);
            for(int i=0;i<4;i++) {
                if(!arr[druhy][i].empty()) {
                    int tmp = arr[druhy][i].back();
                    arr[help(a[counter])][i].push_back(tmp);
                    arr[druhy][i].pop_back();
                    arr[help(a[counter])][druhy].pop_back();
                }
            }
            return 1;
        }
    }
    return 0;
}


int main() {
    cin>>a>>b;
    mySize = a.size();
    counter = mySize - 1;
    for(int i=0;i<a.size();i++){
        int tmp1 = help(a[i]);
        int tmp2 = help(b[i]);
        if(tmp1 == tmp2){
            visited[i] = 1;
        }
        else{
            arr[tmp1][tmp2].push_back(i);
        }
    }
    int res = 0;
    while(!finished()){
        while(removePerfect()){
            res++;
        }
        res += removeNonPerfect();
    }
    cout << res << endl;
    return 0;
}