#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<int>
#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;

ll dfs(vvi &tab, vb &vis, ll d, ll x){
    if(x == d){
        return 0;
    }
    vis[x] = false;
    ll res = -1;
    FOR(i, 0, 4){
        // cout << "try " << x << i << endl;
        if(x != i && vis[i] && tab[x][i] > 0){
            // cout << "connect " << x << i << endl;
            res = dfs(tab, vis, d , i);
            if(res >= 0){
                res++;
                tab[x][i]--;
                break;
            }
        }
    }
    vis[x] = true;
    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]);
            // cout << x << " " << y << endl;
            ll tmp = dfs(tab, vis, x, y);
            // cout << tmp << endl;
            if(tmp > 0){
                res += tmp;
            } else {
                tab[x][y]++;
            }
        }
    }
    cout << res << endl;
    return 0;
}