#include <bits/stdc++.h>
#define NIL (-1)
#define ll long long
using namespace std;
const double PI = acos(-1.0);

int main() {
	int N, M;
	cin >> N >> M;

	vector<int> A(N), B(M);
	for (int i = 0; i < N; i++) cin >> A[i];
	for (int i = 0; i < M; i++) cin >> B[i];

	int ans = -1;
	for (int i = 0; i < N * M; i++) {
		int n, m;
		n = A[i % N];
		m = B[i % M];
		if (n == m) {
			ans = i + 1;
			break;
		}
	}
	cout << ans << endl;

	return 0;
}