func main()
	var d: int :: cui@inputInt()
	var s: []char :: "x".repeat(20) ~ cui@input() ~ cui@input() ~ "x".repeat(20)
	var cnt: int :: 0
	var c: char :: 'x'
	var a: []int :: #[0]int
	for i(0, ^s - 1)
		if(s[i] = c)
			do cnt :+ 1
		else
			do a :~ [cnt]
			do cnt :: 1
			do c :: s[i]
		end if
	end for
	do a :~ [cnt]
	var ans: int :: d
	for i(1, ^a - 1, 2)
		if(a[i - 1] > d | a[i + 1] > d)
			do ans :: [ans, a[i] + d].max()
		end if
		if(i <> 1 & a[i - 1] <= d)
			do ans :: [ans, a[i] + a[i - 1] + a[i - 2]].max()
		end if
	end for
	do cui@print("\{ans}\n")
end func