#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <cmath>
#include <tuple>
#include <bitset>

#define rep(i,n) for(i=0; i<n; ++i)
#define in(a) cin >> a
#define out(a,b) cout << a << b
#define print_vec(v) for(auto it=v.begin();it!=v.end();++it)cout<<*it <<" ";cout<<endl
#define print_vec2d(v) for(int i=0;i<v.size();++i){for(int j=0;j<v[i].size();++j)cout<<v[i][j]<<" ";cout<<endl;}cout<<endl
using namespace std;
using lint = long long;

int main(void){
    lint i, j, k, cnt=0;
    lint p, n, m;
    in(n); in(m);
    vector<int> a(n);
    vector<int> b(m);
    rep(i,n)    in(a[i]);
    rep(i,m)    in(b[i]);
    int cur=0;
    bool f=false;
    while(true){
        if(a[cur%n]==b[cur%m]){
            f=true;
            break;
        }
        cur++;
        if(cur%n==0&&cur%m==0)  break;
    }
    f?out(cur+1,endl):out(-1,endl);
    return 0;
}