#include<iostream>
#include<string>
#include<algorithm>

using namespace std;

int main() {
	int n, m;
	cin >> n >> m;
	string s,t;
	cin >> s>>t;

	int acount1 = 0, acount2 = 0, bcount1 = 0, bcount2 = 0;
	if (n == m) {
		for (int i = 0;i < n;i++) {
			if (s[i] == 'A')acount1++;
			else if (s[i] == 'B')bcount1++;
			if (t[i] == 'A')acount2++;
			else if (t[i] == 'B')bcount2++;
		}
	}
	else {
		for (int i = 0;i < s.size();i++) {
			if (s[i] == 'A')acount1++;
			else if (s[i] == 'B')bcount1++;
		}
		for (int i = 0;i < t.size();i++) {
			if (t[i] == 'A')acount2++;
			else if (t[i] == 'B')bcount2++;
		}
	}
	int ans = min(acount1, acount2) + min(bcount1,bcount2);
	cout << ans<<endl;

	return 0;
}