#include <bits/stdc++.h>

bool debug=true;

#define ll long long
#define ull unsigned long long

#define log(var) if(debug) {cout << #var << ": " << (var) << endl;}
#define tr(container, it) \
	 for(typeof(container.begin() it = container.begin(); it != container.end(); it++)

using namespace std;

int inline code(char x)
{
if (x == 'A')
	return 0;
if (x == 'C')
	return 1;
if (x == 'T')
	return 2;
if (x == 'G')
	return 3;
cout << "fatal error" << endl;
return -1;
}

int main(){
string a, b;
cin >> a >> b;
int n = a.length();
int* counts = new int[16]();
for (int i =0; i < n;i++)
{
	int ac=code(a[i]),bc=code(b[i]);
	if (ac!=bc)
		counts[ac+4*bc]++;
}
int cost=0;
for (int i=0;i<4;i++)
{
	for (int j=i+1;j<4;j++)
	{
		int rem=min(counts[i+4*j],counts[j+4*i]);
		cost+=rem;
		counts[i+4*j]-=rem;
		counts[j+4*i]-=rem;
	}
}
for (int i=0;i<4;i++)
{
	for (int j=0;j<4;j++)
	{
		for (int k=0;k<4;k++)
		{
			if (i==j||j==k||i==k)
				continue;
			int rem=min(min(counts[i+4*j],counts[j+4*k]),counts[k+4*i]);
			cost+=2*rem;
			counts[i+4*j]-=rem;
			counts[j+4*k]-=rem;
			counts[k+4*i]-=rem;
		}
	}
}
for (int i=0;i<16;i++)
{
	if (counts[i]!=0)
	{
		cost+=3*counts[i];
		break;
	}
}
cout << cost << endl;
return 0;
}