#include<iostream>
#include<string>
#include<map>
#include<vector>

using namespace std;

string s, r, x, y;

int pole[4][4];
int ham[4][4];
vector<int> v, u;

char f[] = {'A', 'C', 'G', 'T'};

map<char, int> mp;

int ans = 0;

void xxx()
{
	for(int i = 0; i < 4; i++)
		for(int j = 0; j < 4; j++)
			ham[i][j] = pole[i][j] = 0;
	for(int i = 0; i < s.size(); i++)
	{
		pole[mp[s[i]]][mp[r[i]]]++;

	}
	for(int i = 0; i < 4; i++)
	{
		pole[i][i] = 0;
		for(int j = 0; j < i; j++)
		{
			int mi = min(pole[i][j], pole[j][i]);
			ans += mi;
			pole[i][j] -= mi;
			pole[j][i] -= mi;

		}
	}
	s = r = "";
	for(int i = 0; i < 4; i++)
	{
		for(int j = 0; j < 4; j++)
		{
			for(int k = 0; k < pole[i][j]; k++)
			{
				s += f[i];
				r += f[j];
			}
		}
	}
}

void delej(char a){
	for(int i = 0; i < s.size(); i++)
	{
		if(s[i] == a)
			v.push_back(i);
		if(r[i] == a)
			u.push_back(i);
	}	
	ans += v.size();
	for(int i = 0; i < v.size(); i++)
	{
		swap(s[v[i]], s[u[i]]);
	}
	xxx();
	v.clear();
	u.clear();
}

int main()
{
	cin>>s>>r;
	mp['A'] = 0;
	mp['C'] = 1;
	mp['G'] = 2;
	mp['T'] = 3;
	xxx();
	delej('A');
	delej('C');
	delej('G');
	cout<<ans<<endl;
	return 0;
}