#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

typedef vector<vector<int>> graph;

struct edge { int a, b; };
typedef vector<edge> path;

int lettr_code(char c) {
	switch(c) {
		case 'A': return 0;
		case 'C': return 1;
		case 'T': return 2;
		case 'G': return 3;
	}
}

bool check_path(const graph& g, const path& p) {
	for (edge e : p) {
		if (g[e.a][e.b] == 0) return false;
	}

	return true;
}

void rm_path(graph& g, const path& p) {
	for (edge e : p) {
		g[e.a][e.b]--;
	}
}

int main() {
	ios_base::sync_with_stdio(0);

	string a, b;
	cin >> a >> b;

	int n = a.size();
	graph g(4, vector<int>(4, 0));
	
	for (int i = 0; i < n; i++) {
		if (a[i] == b[i]) continue;
		g[lettr_code(a[i])][lettr_code(b[i])]++;
	}

	vector<path> paths;
	
	vector<int> cycle(4);
	cycle[0] = 0;
	cycle[1] = 1;
	cycle[2] = 2;
	cycle[3] = 3;

	do {
		for (int i = 2; i <= 4; i++) {
			path p;
			for (int j = 0; j < i-1; j++) {
				p.push_back({ cycle[j], cycle[j+1] });
			}
			p.push_back({ cycle[i-1], cycle[0] });
			paths.push_back(p);
		}
	} while (next_permutation(cycle.begin(), cycle.end()));

	int swaps = 0;

	for (path& p : paths) {
		while (check_path(g, p)) {
			rm_path(g, p);
			swaps += p.size() - 1;
		}
	}

	cout << swaps << endl;

	return 0;
}