#include <bits/stdc++.h>

using namespace std;

typedef pair<int, int> pii;
typedef long long ll;
const int N = 2000010, MOD = 998244353, INF = 0x3f3f3f3f;
int n, m, w[N];

int a[N];
bool st[N];
void solve() {
    scanf("%d%d", &n, &m);
    for (int i = 0; i < 2 * n + 1; i++) scanf("%d", a + i);
    queue<int> q;
    for (int i = 1; i < m + 1; i++) scanf("%d", w + i), q.push(w[i]);
    while (q.size()) {
        auto u = q.front(); q.pop();
        if (st[u]) continue;
        st[u] = 1;
        for (int i = 1; i < m + 1; i++) q.push(a[u + w[i]]);
    }
    int res = 0;
    for (int i = 0; i <= 2 * n; i++) res += st[i];
    printf("%d\n", res);
}

int main() {
    int T = 1;
    // cin >> T;
    while (T--) solve();
    return 0;
}