#include "bits/stdc++.h"
#include <algorithm>
#include <ios>
#include <vector>
using namespace std;

int N, M;
vector<int> telj;
vector<int> hata;

int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    cin >> N >> M;
    telj.resize(N);
    for (int i = 0; i < N; i++) {
        cin >> telj[i];
    }
    hata.resize(M);
    for (int i = 0; i < M; i++) {
        cin >> hata[i];
        hata[i]--;
    }
    int r = 0;
    for (int h : hata) {
        int i = h;
        while (i != -1 && telj[i] == 0) i--;
        if (i != -1) {
            r++;
            telj[i]--;
        }
    }
    cout << r << endl;
}