#include <iostream>
#include <vector>

using namespace std;

int main()
{
    int N, M;
    cin >> N >> M;
    vector<int> A(N), B(N);
    for (int i = 0; i < N; i++) cin >> A[i];
    for (int i = 0; i < M; i++) cin >> B[i];
    
    int64_t index = -1;
    
    for (int64_t i = 0; i < N * M; i++)
    {
        int64_t j = i % N;
        int64_t k = i % M;
        if (A[j] == B[k]) {
            index = i;
            break;
        }
    }
    if (index >= 0) index++;
    
    cout << index << std::endl;

    return 0;
}