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

int N, M;
vector<int> telj;
vector<int> hata;
int r = 0, a;

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(N);
    for (int i = 0; i < M; i++) {
        cin >> a;
        hata[a-1]++;
    }
    int i = 0, j = 0;
    while (i != N && j != N) {
        while (i < N && telj[i] == 0) i++;
        while (j < N && hata[j] == 0) j++;
        if (i > j) {
            j++;
            continue;
        }
        if (j >= N) break;
        hata[j]--;
        telj[i]--;
        r++;
    }
    cout << r << endl;
}