#include <iostream>
#include <algorithm>
#include <vector>
#include <unordered_map>

using namespace std;

vector<int> OOO[4][4];
int cnt[4][4];

int main(){
	string s, t;
	cin >> s >> t;
	unordered_map <char, int> ump;
	ump['A'] = 0;
	ump['C'] = 1; // (1, 3, 0)
	ump['G'] = 2;
	ump['T'] = 3;
	int n = s.size();
	int m = t.size();
	for(int i = 0; i < n; i++){
		if(s[i] != t[i]){
			OOO[ump[s[i]]][ump[t[i]]].push_back(i);
			cnt[ump[s[i]]][ump[t[i]]]++;
		}
	}
	for(int i = 0; i < 4; i++){
		for(int j = 0; j < 4; j++){
			if(i == j) continue;
			reverse(OOO[i][j].begin(),OOO[i][j].end());
		}
	}
	int c1 = 0;
	for(int i = 0; i < n; i++){
		if(s[i] != t[i]){
			if(OOO[ump[t[i]]][ump[s[i]]].size() && OOO[ump[s[i]]][ump[t[i]]].size()){
				int x = OOO[ump[t[i]]][ump[s[i]]].back();
				OOO[ump[t[i]]][ump[s[i]]].pop_back();
				OOO[ump[s[i]]][ump[t[i]]].pop_back();
				cnt[ump[s[i]]][ump[t[i]]]--;
				cnt[ump[t[i]]][ump[s[i]]]--;
				c1++;
				swap(s[i], s[x]);
			}
		}
	}
	for(int i = 0; i < 4; i++){
		for(int j = 0; j < 4; j++){
			//cout << cnt[i][j] << " ";
		}
		//cout << endl;
	}
	int res = c1;
	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(j == k) continue;
				int mn = min({cnt[i][j], cnt[j][k], cnt[k][i]});
				if(i == 1 && j == 3 && k == 0){
					//cout << cnt[i][j] << " " << cnt[j][k] << " " << cnt[k][i] << endl;
				}
				if(mn > 0){
					//cout << mn << endl;
					res += mn * 2;
					cnt[i][j] -= mn;
					cnt[j][k] -= mn;
					cnt[k][i] -= mn;
				}
			}
		}
	}
	
	for(int i = 0; i < 4; i++){
		for(int j = 0; j < 4; j++){
			//cout << cnt[i][j] << " ";
		}
		//cout << endl;
	}
	
	
	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(j == k) continue;
				for(int l = 0; l < 4; l++){
					if(k == l) continue;
					int mn = min({cnt[i][j], cnt[j][k], cnt[k][l], cnt[l][i]});
					if(mn > 0){
						res += mn * 3;
						cnt[i][j] -= mn;
						cnt[j][k] -= mn;
						cnt[k][l] -= mn;
						cnt[l][i] -= mn;
					}
				}
			}
		}
	}
	cout << res << endl;
	//cout << s << " " << t << " " << c2 << endl;
	//cout << c1 + (2 * c2 / 3) + (c2 % 3) << endl;
	
}