package main

import (
	"fmt"
	"math"
)

func main() {
	var d int
	_, _ = fmt.Scan(&d)

	c := make([]int, 0)
	for i := 0; i < 2; i++ {
		line := ""
		_, _ = fmt.Scan(&line)
		for _, s := range line {
			if string(s) == "x" {
				c = append(c, 0)
			} else {
				c = append(c, 1)
			}
		}
	}

	max := 0
	for i := 0; i < 14; i++ {
		if c[i] == 0 {
			tmp := make([]int, 14)
			copy(tmp, c)
			for j := i; j < int(math.Min(float64(i+d), 14)); j++ {
				tmp[j] = 1
			}
			// fmt.Println(tmp)

			cnt := 0
			for j := 0; j < 14; j++ {
				cnt += tmp[j]
				if tmp[j] == 0 || j == 13 {
					if max < cnt {
						max = cnt
					}
					cnt = 0
				}
			}
		}
	}

	fmt.Println(max)
}