#include <bits/stdc++.h>

using namespace std;

#define FOR(a, b, c) for(int a = (b); a < (c); a++)
#define SZ(a) (int)((a).size())
#define SIZE(a) (int)((a).size())
#define ALL(a) (a).begin(), (a).end()
using ll = long long;
#define LL long long
#define PB push_back

using pch = pair<char, char>;

int main() {
    ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    string s, t;
    cin >> s >> t;
    int res = 0;
    map<pch, int> M;
    int n = SZ(s);
    FOR(i, 0, n) {
        if(s[i] != t[i]) {
            M[{s[i], t[i]}]++;
        }
    }

    string wsz = "ACGT";

    FOR(i, 0, 4) {
        FOR(j, i + 1, 4) {
            pch x = {wsz[i], wsz[j]};
            pch y = {wsz[j], wsz[i]};
            int r = min(M[x], M[y]);
            M[x] -= r;
            M[y] -= r;
            res += r;
        }
    }

    for(auto dl : {3, 4}) {
        sort(ALL(wsz));
        do {
            int r = 1e9;
            FOR(i, 0, dl - 1) {
                r = min(r, M[{wsz[i], wsz[i + 1]}]);
            }
            r = min(r, M[{wsz[dl - 1], wsz[0]}]);

            FOR(i, 0, dl - 1) {
                M[{wsz[i], wsz[i + 1]}] -= r;
            }
            M[{wsz[dl - 1], wsz[0]}] -= r;

            res += r * (dl - 1);
        } while(next_permutation(ALL(wsz)));
    }

    cout << res << "\n";
}