#include <bits/stdc++.h>
using namespace std;
#define REP(i,n) for (int i = 0; i < (n); ++i)
#define DREP(i,s,n) for(int i = (s); i < (n); i++)
template<class T> inline bool chmin(T& a, T b) {if (a > b) {a = b;return true;}return false;}
template<class T> inline bool chmax(T& a, T b) {if (a < b) {a = b;return true;}return false;}
using ll = long long;
using P = pair<int,int>;
using Pl = pair<long long,long long>;
using veci = vector<int>;
using vecl = vector<long long>;
using vecveci = vector<vector<int>>;
using vecvecl = vector<vector<long long>>;
const int MOD = 1000000007;
const double pi = acos(-1);
ll gcd(ll a, ll b) {if(b == 0) return a; else return gcd(b,a%b);}
ll lcm(ll a, ll b) {return a*b/gcd(a,b);}

void ex_gcd(ll a, ll b, ll &x, ll &y) {
    if(b == 0) {
        x = 1, y = 0;
        return;
    }
    ex_gcd(b,a%b,y,x);
    y -= (a/b)*x;
}

int main() {
    int N,M; cin >> N >> M;
    veci A(N),B(M);
    REP(i,N) cin >> A[i];
    REP(i,M) cin >> B[i];
    REP(i,M-1) REP(j,N) A.push_back(A[j]);
    REP(i,N-1) REP(j,M) B.push_back(B[j]);
    REP(i,N*M) if(A[i] == B[i]) {
        cout << i+1 << endl;
        return 0;
    }
    cout << -1 << endl;
    return 0;
}