#include <iostream>

using namespace std;

int mat[4][4];

int num[256];

int main() {
	num['C'] = 0;
	num['G'] = 1;
	num['A'] = 2;
	num['T'] = 3;

	string a, b;
	cin>>a>>b;
	int n = a.size();
	for (int i = 0; i < n; ++i) {
		mat[num[a[i]]][num[b[i]]]++;
	}
	
	int sol = 0;
	for (int i = 0; i < 4; ++i) {
		for (int j = 0; j < 4; ++j) if (j != i) {
			int x = min(mat[i][j], mat[j][i]);
			sol += x;
			mat[i][j] -= x;
			mat[j][i] -= x;
		}
	}
	for (int i = 0; i < 4; ++i) {
		for (int j = 0; j < 4; ++j) if (j != i) {
			for (int k = 0; k < 4; ++k) if (k != i && k != j) {
				int x = min(mat[i][j], min(mat[j][k], mat[k][i]));
				sol += 2 * x;
				mat[i][j] -= x;
				mat[j][k] -= x;
				mat[k][i] -= x;
			}
		}
	}
	for (int i = 0; i < 4; ++i) {
		for (int j = 0; j < 4; ++j) if (j != i) {
			for (int k = 0; k < 4; ++k) if (k != i && k != j) {
				for (int l = 0; l < 4; ++l) if (l != i && l != j && l != k) {
					int x = min(mat[i][j], min(mat[j][k], min(mat[k][l], mat[l][i])));
					sol += 3 * x;
					mat[i][j] -= x;
					mat[j][k] -= x;
					mat[k][l] -= x;
					mat[l][i] -= x;
				}
			}
		}
	}
	cout<<sol<<endl;
}