#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using ull = unsigned long long;

#define rep(i,n) for(ll i=0;i<n;++i)
#define all(a) (a).begin(),(a).end()
ll intpow(ll a, ll b){ ll ans = 1; while(b){ if(b & 1) ans *= a; a *= a; b /= 2; } return ans; }
ll modpow(ll a, ll b, ll p){ ll ans = 1; while(b){ if(b & 1) (ans *= a) %= p; (a *= a) %= p; b /= 2; } return ans; }
template<class T> T div_floor(T a, T b) { return a / b - ((a ^ b) < 0 && a % b); }
template<class T> T div_ceil(T a, T b) { return a / b + ((a ^ b) > 0 && a % b); }
template <typename T, typename U> inline bool chmin(T &x, U y) { return (y < x) ? (x = y, true) : false; }
template <typename T, typename U> inline bool chmax(T &x, U y) { return (x < y) ? (x = y, true) : false; }

template<typename T>
ostream &operator<<(ostream &os, const vector<T> &a){
    if (a.empty()) return os;
    os << a.front();
    for (auto e : a | views::drop(1)){
        os << ' ' << e;
    }
    return os;
}

void dump(auto ...vs){
    ((cout << vs << ' '), ...) << endl;
}



void solve() {
    ll N,M;
    cin>>N>>M;
    vector<ll> A(2*N+1);
    rep(i,2*N+1){
        cin>>A[i];
    }
    vector<ll> S(M);
    rep(i,M)cin>>S[i];
    vector<bool> used(N+1,false);
    queue<ll> qu;
    rep(i,M){
        used[S[i]]=true;
        qu.push(S[i]);
    }
    while (qu.size()>0){
        ll cp=qu.front();qu.pop();
        for (ll sp:S){
            ll np=A[cp+sp];
            if (!used[np]){
                used[np]=true;
                qu.push(np);
            }
        }
    }
    cout<<count(all(used),true)<<'\n';
    return;
}


int main() {
    cin.tie(0)->sync_with_stdio(0);
    ll T=1;
    while (T--){
        solve();
    }
    return 0;
}