#include <iostream>
using namespace std;
int main(void) {
	int n, m;
	cin >> n >> m;
	int a = 0, b = 0;
	char c;
	for (int i = 0; i < n; ++i) {
		cin >> c;
		if (c == 'A') ++a;
		else ++b;
	}
	int ans = 0;
	for (int i = 0; i < m; ++i) {
		cin >> c;
		if (c == 'A' && a > 0)
			++ans, --a;
		if (c == 'B' && b > 0)
			++ans, --b;
	}
	cout << ans << endl;
	return 0;
}