//Normal

#define _GLIBCXX_DEBUG
#define ll long long
#include <bits/stdc++.h>
using namespace std;
using Graph = vector<vector<int>>;

void print(auto a){
  cout << a;
}

void prints(auto a){
  cout << a << " ";
}

void prints(){
  cout << " ";
}

void printl(auto a){
  cout << a << endl;
}

void printl(){
  cout << endl;
}

void fix(int n){
  cout << fixed << setprecision(n);
}

int siz(string s){
  return (int)s.size();
}

int main(){
  int N, M; cin >> N >> M;
  vector<int> A(N), B(M);
  map<int, int> mp;
  for(int i = 1; i <= N; i++){ 
    cin >> A[i%N];
    mp[A[i%N]]++;
  }

  bool flag = false;
  for(int i = 1; i <= M; i++){
    cin >> B[i%M];

    if(mp[B[i%M]] > 0){
      flag = true;
    }
  }

  if(!flag){
    printl(-1);
  }else{
    for(int i = 1; i <= N*M; i++){
      if(A[i%N] == B[i%M]){
        printl(i);

        break;
      }
    }
  }

  return 0;
  
}