#include <bits/stdc++.h>

using namespace std;
using ll = long long;

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];
    for (int i=0; i<N*M; i++){
        if (A[i%N] == B[i%M]){
            cout << i+1 << endl;
            return 0;
        }
    }

    cout << -1 << endl;

    return 0;
}