import java.util.Scanner;

class Codechef {
    public static void main(String[] args) throws java.lang.Exception {
        Scanner sc = new Scanner(System.in);

        int t = sc.nextInt();

        sc.nextLine();
        String a = sc.nextLine();
        String b = sc.nextLine();

        int counter = 0;
        for (int i = 0; i < t; i++) {
            if (a.charAt(i) != b.charAt(i)) {
                counter++;
            }
        }
        System.out.println(counter % 2 == 0 ? counter / 2 : "NO");
    }
}