import java.util.Scanner;

public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int m = sc.nextInt();
		int[] a = new int[n];
		for (int i = 0; i < n; i++) {
			a[i] = sc.nextInt();
		}
		int[] b = new int[m];
		for (int i = 0; i < m; i++) {
			b[i] = sc.nextInt();
		}
		sc.close();

		int nm = n * m;
		for (int i = 0; i < nm; i++) {
			if (a[i % n] == b[i % m]) {
				System.out.println(i + 1);
				return;
			}
		}
		System.out.println(-1);
	}
}