#include <bits/stdc++.h>

using namespace std;
typedef long long ll;

int to_num(char c) {
    switch(c) {
        case 'A':
            return 0;
        case 'C':
            return 1;
        case 'G':
            return 2;
        case 'T':
            return 3;
    }
}

int main() {
    string from, to;
    cin >> from >> to;
    multiset<char> basesFrom[4];
    multiset<char> basesTo[4];
    for(int i = 0; i < from.size(); i++) {
        if(from[i] != to[i]) {
            basesFrom[to_num(from[i])].insert(to_num(to[i]));
            basesTo[to_num(to[i])].insert(to_num(from[i]));
        }
    }
    ll counter = 0;
    for(int from = 0; from < 4; from++) {
            for (int to = 0; to < 4; to++) {
                if(from != to) {
                    while(basesFrom[from].find(to) != basesFrom[from].end() &&
                          basesFrom[to].find(from) != basesFrom[to].end()
                            ) {
                        basesFrom[from].erase(basesFrom[from].find(to));
                        basesFrom[to].erase(basesFrom[to].find(from));
                        counter++;
                    }
                }
        }
    }
    for(int from = 0; from < 4; from++) {
        for(int over = 0; over < 4; over++) {
            for (int to = 0; to < 4; to++) {
                if(from != over && over != to && from != to) {
                    while(basesFrom[from].find(over) != basesFrom[from].end() &&
                          basesFrom[over].find(to) != basesFrom[over].end() &&
                          basesFrom[to].find(from) != basesFrom[to].end()
                    ) {
                        basesFrom[from].erase(basesFrom[from].find(over));
                        basesFrom[over].erase(basesFrom[over].find(to));
                        basesFrom[to].erase(basesFrom[to].find(from));
                        counter+= 2;
                    }
                }
            }
        }
    }
    ll left = 0;
    for(int from = 0; from < 4; from++) {
        left += basesFrom[from].size();
    }
    counter += 3*(left / 4);
    cout << counter << endl;
}