#include <bits/stdc++.h>
#define rep(i,a,b) for (int i=a; i<b; i++)
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define all(x) (x).begin(),(x).end()
#define debug printf
using namespace std;
typedef pair <int, int> PII;
typedef vector <int> VI;
typedef long long ll;
typedef long double ld;

int in[4], out[4], t[4][4];

int f(char a){
	if(a == 'A') return 0;
	if(a == 'C') return 1;
	if(a == 'T') return 2;
	if(a == 'G') return 3;
}

int main ()
{
	ios_base::sync_with_stdio(false);
	string s,d; cin>>s>>d;
	int n = s.size();
	int a[n], b[n];
	for(int i=0;i<n;i++){
		a[i] = f(s[i]);
		b[i] = f(d[i]);
		t[a[i]][b[i]]++;
	}
	int ans = 0;
	for(int i=0;i<4;i++) for(int j=0;j<4;j++){
		if(i==j) continue;
		int x = min(t[i][j], t[j][i]);
		ans+=x;
		t[i][j] -= x;
		t[j][i] -= x;
	}
	for(int i=0;i<4;i++) for(int j=0;j<4;j++){
		if(i==j) continue;
		if(t[i][j] == 0) continue;
		in[i] ++;
		out[j] ++;
	}
	int I = -1, J = -1;
	for(int i=0;i<4;i++) for(int j=0;j<4;j++){
		if(i==j) continue;
		if(t[i][j] == 0) continue;
		if(in[i] == 1 && out[j] == 1 && t[i][j] != 0){
			ans+= 2*t[i][j];
			I = i; J = j; i = 5; j = 5;
		}
	}
	if(I != -1){
		for(int i=0;i<4;i++) for(int j=0;j<4;j++){
			if(i == j || i == I || j == I || i == J || j == J) continue;
			ans += max(t[i][j], t[j][i]);
			cout<<ans;
			return 0;
		}
	}
	cout<<ans;
	
	
}