import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int D = sc.nextInt();
		String string = sc.next() + sc.next();
		string = string.replaceAll("o", "1").replaceAll("x", "0");

		int max = 0;
		for (int i = 0; i < (1 << 14); i++) {
			if (Integer.bitCount(i & Integer.parseInt(string, 2)) != Integer.bitCount(Integer.parseInt(string, 2))) {
				continue;
			}
			if (Integer.bitCount(i) > Integer.bitCount(Integer.parseInt(string, 2)) + D) {
				continue;
			}

			char[] bits = ("0" + Integer.toBinaryString(i) + "0").toCharArray();
			int cnt = 0;
			for (int j = 0; j < bits.length; j++) {
				if (bits[j] == '1') {
					cnt++;
				} else {
					max = Math.max(max, cnt);
					cnt = 0;
				}
			}


		}
		System.out.println(max);
	}
}