#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>

int main(void) {
	int i,n;
	char *ip, *np;
	scanf("%d %d",&i,&n);

	if ((i*n == 0)||(i>10000)||(n>10000)) {
		printf("!check undefined number!\n");
		return 1;
	}

	ip = (char*)malloc(i);
	if (ip == NULL) {
		printf("メモリエラー\n");
	}
	np = (char*)malloc(n);
	if (np == NULL) {
		printf("メモリエラー\n");
	}
	scanf("%s", ip);
	scanf("%s", np);

	int a_up = 0;//上の服A
	int b_up = 0;//上の服B

	int a_buttom = 0;//下の服A
	int b_buttom = 0;//下の服B
	for (int x = 0; x < i;x++) {
		if (*(ip+x)=='A') {
			a_up++;
		}
		else {
			b_up++;
		}
	}
	for (int x = 0; x < n; x++) {
		if (*(np + x) == 'A') {
			a_buttom++;
		}
		else {
			b_buttom++;
		}
	}
	int combi = 0;
	if (a_up < a_buttom) {
		combi += a_up;
	}
	else {
		combi += a_buttom;
	}
	if (b_up < b_buttom) {
		combi += b_up;
	}
	else {
		combi += b_buttom;
	}
	printf("%d", combi);
	return 0;
}