#include <bits/stdc++.h>

using namespace std;

#define fio ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);

typedef long long ll;
string s1,s2;
ll res;
vector<vector<int> > V(4,vector<int>(4,0));
int main() { fio
    cin>>s1>>s2;
    for(int i=0;i<s1.size();i++){
        if(s1[i]==s2[i])continue;
        int a=0,b=0;
        if(s1[i]=='C')a=1;
        if(s1[i]=='G')a=2;
        if(s1[i]=='T')a=3;

        if(s2[i]=='C')b=1;
        if(s2[i]=='G')b=2;
        if(s2[i]=='T')b=3;
        V[a][b]++;
    }
    for(int i=0;i<4;i++){
        for(int j=i+1;j<4;j++){
            ll kolko=min(V[i][j],V[j][i]);
            res+=kolko;
            V[i][j]-=kolko;
            V[j][i]-=kolko;
        }
    }
    for(int i=0;i<4;i++){
        for(int j=0;j<4;j++){
            if(i==j)continue;
            for(int k=0;k<4;k++) {
                if(i==k || j==k)continue;
                ll kolko = min(min(V[i][j], V[j][k]),V[k][i]);
                res += kolko*2LL;
                V[i][j] -= kolko;
                V[j][k] -= kolko;
                V[k][i] -= kolko;
            }
        }
    }
    int sucet=0;
    for(int i=0;i<4;i++){
        for(int j=0;j<4;j++)sucet+=V[i][j];
    }
    res+=sucet/4*3;
    cout <<res<<endl;
    return 0;
}