#include <bits/stdc++.h>
using namespace std;

#define st first
#define nd second

constexpr int maxn = 1000 * 1000 + 5;

int A[maxn], B[maxn];
int mapping[500];
int cnt[4][4];

int res = maxn * 5;

void czworki(int wyn) {
    for(int i = 1; i < 4; i++) {
        if(cnt[0][i] > 0) {
            res = min(res, wyn + cnt[0][i] * 3);
            return;
        }
    }
    res = min(res, wyn);
}

void trojki(int wyn) {
    vector<pair<int, pair<int, int>>> tr;
    for(int a = 0; a < 4; a++) {
        for(int b = 0; b < 4; b++) {
            for(int c = 0; c < 4; c++) {
                if(a == c or a == b or b == c) continue;
                if(cnt[c][a] == 0 or cnt[a][b] == 0 or cnt[b][c] == 0)
                    continue;
                tr.emplace_back(a, pair<int, int>(b, c));
            }
        }
    }

    do {
        int tmp[4][4];
        for(int i = 0; i < 4; i++) {
            for(int j = 0; j < 4; j++) {
                tmp[i][j] = cnt[i][j];
            }
        }

        int _wyn = wyn;
        for(auto t : tr) {
            int a, b, c;
            a = t.st;
            tie(b, c) = t.nd;
            auto x = min({cnt[a][b], cnt[b][c], cnt[c][a]});
            cnt[a][b] -= x;
            cnt[b][c] -= x;
            cnt[c][a] -= x;
            _wyn += x * 2;
        }

        czworki(_wyn);

        for(int i = 0; i < 4; i++) {
            for(int j = 0; j < 4; j++) {
                cnt[i][j] = tmp[i][j];
            }
        }
    } while(next_permutation(tr.begin(), tr.end()));
}

void pary() {
    vector<pair<int, int>> pary;
    for(int i = 0; i < 4; i++) {
        for(int j = i + 1; j < 4; j++) {
            pary.emplace_back(i, j);
        }
    }

    int wyn = 0;
    do {
        int tmp[4][4];
        for(int i = 0; i < 4; i++) {
            for(int j = 0; j < 4; j++) {
                tmp[i][j] = cnt[i][j];
            }
        }
        for(auto p : pary) {
            auto x = min(cnt[p.st][p.nd], cnt[p.nd][p.st]);
            cnt[p.st][p.nd] -= x;
            cnt[p.nd][p.st] -= x;
            wyn += x;
        }

        trojki(wyn);

        for(int i = 0; i < 4; i++) {
            for(int j = 0; j < 4; j++) {
                cnt[i][j] = tmp[i][j];
            }
        }
    } while(next_permutation(pary.begin(), pary.end()));
}

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    mapping['A'] = 0;
    mapping['C'] = 1;
    mapping['T'] = 2;
    mapping['G'] = 3;

    int n;
    for(auto* t : {A, B}) {
        string s;
        cin >> s;
        n = s.size();
        for(int i = 0; i < s.size(); i++) {
            t[i] = mapping[s[i]];
        }
    }

    for(int i = 0; i < n; i++) {
        if(A[i] != B[i])
            cnt[A[i]][B[i]]++;
    }

    pary();

    cout << res << endl;
}