#include<bits/stdc++.h>
#define ll long long
#define ld long double
#define fi first
#define se second
#define pll pair<ll, ll>
#define INF (1ll << 60)
#define ppl pair<pll, pll>
#define vl vector<ll>
#define vi vector<ll>
#define vvi vector<vi>
#define vvl vector<vl>
#define vb vector<bool>
#define FOR(i,a,b) for(ll i=a;i<b;++i)
#define pb push_back
#define ALL(X) X.begin(),X.end()
#define endl (char)10
using namespace std;

vi dfs(vvi &tab, vb &vis, ll g, ll x, ll d){
    if(d >= g){
        return vi(0);
    }
    if(!vis[x]){
        return vi(1, x);
    }
    vis[x] = false;
    FOR(i, 0, 4){
        if(x != i && tab[x][i] > 0){
            vi res = dfs(tab, vis, g, i, d+1);
            if(res.size() > 0){
                res.push_back(x);
                vis[x] = true;
                return res;
            }
        }
    }
    vis[x] = true;
    return vi(0);
}

ll fix(vvi &tab, vi &v){
    ll min_v = INF;
    ll res = 0;
    FOR(i, 0, v.size()-1){
        int s = v[i+1];
        int e = v[i];
        min_v = min(min_v, tab[s][e]);
    }
    FOR(i, 0, v.size()-1){
        int s = v[i+1];
        int e = v[i];
        tab[s][e] -= min_v;
        res += min_v;
    }
    res -= min_v;
    return res;
}


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

int main() {
    ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
    string in, out;
    cin >> in >> out;
    vb vis(4, true);
    vvi tab(4, vi(4, 0));
    ll res = 0;
    FOR(i, 0, in.size()){
        if(in[i] != out[i]){
            int x = c_to_int(in[i]);
            int y = c_to_int(out[i]);
            tab[x][y]++;
        }
    }
    FOR(d, 1, 6){
        FOR(i, 0, 4){
            bool check = true;
            while(check){
                check = false;
                vi r = dfs(tab, vis, d, i, 0);
                if(r.size() > 0){
                    res += fix(tab, r);
                }
            }
        }
    }
    cout << res << endl;
    return 0;
}