import java.io.*;
import java.util.*;


public class Main_yukicoder717 {

	private static Scanner sc;
	private static Printer pr;

	private static void solve() {
		int n = sc.nextInt();
		int m = sc.nextInt();
		
		char[] s = sc.next().toCharArray();
		char[] t = sc.next().toCharArray();

		int cnts = 0;
		for (char c : s) {
			if (c == 'A') {
				cnts++;
			}
		}
		int cntt = 0;
		for (char c : t) {
			if (c == 'A') {
				cntt++;
			}
		}

		pr.println(Math.min(cnts, cntt) + Math.min(n - cnts, m - cntt));
	}

	// ---------------------------------------------------
	public static void main(String[] args) {
		sc = new Scanner(INPUT == null ? System.in : new ByteArrayInputStream(INPUT.getBytes()));
		pr = new Printer(System.out);

		solve();

//		pr.close();
		pr.flush();
//		sc.close();
	}

	static String INPUT = null;

	private static class Printer extends PrintWriter {
		Printer(OutputStream out) {
			super(out);
		}
	}
}