#include <bits/stdc++.h>
using namespace std;

typedef long long LL;
typedef long double LD;
typedef pair < int, int > PII;
typedef pair < LL, LL > PLL;

#define all(x) (x).begin(), (x).end()
#define sz(x) (int)(x).size()

int cnt[4][4];

char MP[256];

int main()
{
	ios_base::sync_with_stdio(0); cin.tie(0);

	MP['C'] = 1;
	MP['G'] = 2;
	MP['T'] = 3;

	int n;
	string s, t;
	cin >> s >> t;
	
	n = sz(s);
	
	for(int i = 0; i < n; i++)
	{
		if(s[i] != t[i])
			cnt[MP[s[i]]][MP[t[i]]]++;
	}
	
	int ans = 0;
	
	for(int i = 0; i < 4; i++)
	{
		for(int j = i + 1; j < 4; j++)
		{
			int x = min(cnt[i][j], cnt[j][i]);
			ans += x;
			cnt[i][j] -= x;
			cnt[j][i] -= x;
		}
	}
	
	for(int i = 0; i < 4; i++)
	{
		for(int j = 0; j < 4; j++)
		{
			for(int k = 0; k < 4; k++)
			{
				if(i != j && i != k && j != k)
				{
					int x = min({cnt[i][j], cnt[j][k], cnt[k][i]});
					ans += 2 * x;
					cnt[i][j] -= x;
					cnt[j][k] -= x;
					cnt[k][i] -= x;
				}		
			}
		}
	}
	
	int tmp = 0;
	for(int i = 0; i < 4; i++)
		for(int j = 0; j < 4; j++)
			tmp += cnt[i][j];
	assert(tmp % 4 == 0);
	
	ans += (tmp / 4) * 3;
	
	cout << ans << "\n";
	
	return 0;
}