#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;
	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);
		}
	}
	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();
				c1++;
				swap(s[i], s[x]);
			}
		}
	}
	int c2 = 0;
	for(int i = 0; i < 4; i++){
		for(int j = 0; j < 4; j++){
			if(i == j) continue;
			c2 += OOO[i][j].size();
		}
	}
	//cout << s << " " << t << " " << c2 << endl;
	cout << c1 + (2 * c2 / 3) + (c2 % 3) << endl;
}