import java.io.IOException;
import java.util.InputMismatchException;
import java.util.LinkedList;
import java.util.List;

public class Main {
	void run() {
		MyScanner sc = new MyScanner();

		int d = sc.nextInt();
		char[] c = (sc.next() + sc.next() + '@').toCharArray();

		int cnt = 1;
		List<Integer> list = new LinkedList<Integer>();
		for (int i = 1; i < c.length; i++) {
			if (c[i] != c[i - 1]) {
				if (c[i - 1] == 'o') {
					list.add(cnt);
				} else {
					list.add(-cnt);
				}
				cnt = 0;
			}
			cnt++;
		}
		Integer[] a = list.toArray(new Integer[0]);

		int max = d;
		for (int i = 0; i < a.length; i++) {
			if (a[i] < 0) {
				if (-a[i] <= d) {
					int tmp1 = 0, tmp2 = 0;
					if (0 <= i - 1) {
						tmp1 = a[i - 1];
					}
					if (i + 1 < a.length) {
						tmp2 = a[i + 1];
					}
					max = Math.max(max, tmp1 + -a[i] + tmp2);
				} else {
					if (0 <= i - 1) {
						max = Math.max(max, a[i - 1] + d);
					}
					if (i + 1 < a.length) {
						max = Math.max(max, a[i + 1] + d);
					}
				}
			} else {
				max = Math.max(max, a[i]);
			}
		}
		System.out.println(max);
	}

	public static void main(String[] args) {
		new Main().run();
	}

	public void mapDebug(int[][] a) {
		System.out.println("--------map display---------");
		for (int i = 0; i < a.length; i++) {
			for (int j = 0; j < a[i].length; j++) {
				System.out.printf("%3d ", a[i][j]);
			}
			System.out.println();
		}
		System.out.println("----------------------------" + '\n');
	}

	class MyScanner {
		int read() {
			try {
				return System.in.read();
			} catch (IOException e) {
				throw new InputMismatchException();
			}
		}

		boolean isSpaceChar(int c) {
			return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
		}

		boolean isEndline(int c) {
			return c == '\n' || c == '\r' || c == -1;
		}

		int nextInt() {
			return Integer.parseInt(next());
		}

		int[] nextIntArray(int n) {
			int[] array = new int[n];
			for (int i = 0; i < n; i++)
				array[i] = nextInt();
			return array;
		}

		long nextLong() {
			return Long.parseLong(next());
		}

		long[] nextLongArray(int n) {
			long[] array = new long[n];
			for (int i = 0; i < n; i++)
				array[i] = nextLong();
			return array;
		}

		double nextDouble() {
			return Double.parseDouble(next());
		}

		double[] nextDoubleArray(int n) {
			double[] array = new double[n];
			for (int i = 0; i < n; i++)
				array[i] = nextDouble();
			return array;
		}

		String next() {
			int c = read();
			while (isSpaceChar(c))
				c = read();
			StringBuilder res = new StringBuilder();
			do {
				res.appendCodePoint(c);
				c = read();
			} while (!isSpaceChar(c));
			return res.toString();
		}

		String[] nextStringArray(int n) {
			String[] array = new String[n];
			for (int i = 0; i < n; i++)
				array[i] = next();

			return array;
		}

		String nextLine() {
			int c = read();
			while (isEndline(c))
				c = read();
			StringBuilder res = new StringBuilder();
			do {
				res.appendCodePoint(c);
				c = read();
			} while (!isEndline(c));
			return res.toString();
		}
	}
}