import java.util.*;
import java.io.*;
import java.math.*;
// import java.util.stream.Stream;

class Main{
    static BufferedReader buff = new BufferedReader(new InputStreamReader(System.in));
    static StringTokenizer st;
    static PrintWriter output = new PrintWriter(System.out);
    static Scanner sc = new Scanner(System.in);
    public static void main(String[] args) throws IOException{
        int n = sc.nextInt();
        int m = sc.nextInt();
        int [] a = new int[n];
        int [] b = new int[m];
        for(int i=0;i<n;i++) a[i] = sc.nextInt();
        for(int i=0;i<m;i++) b[i] = sc.nextInt();
        int ans = -1;
        for(int i=0;i<n*m;i++) {
            if(a[i%n] == b[i%m]) {
                ans = i+1;
                break;
            }
        }
        output.print(ans);
        output.flush();
    }
}