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

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

int main() {
    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]--;
    }
    sort(hata.begin(), hata.end());
    int t = 0, i = 0, r = 0;
    for (int h : hata) {
        if (i == telj[t]) {
            i = 0;
            t++;
            while (telj[t] == 0) t++;
        }
        if (h >= t) {
            i++;
            r++;
        }
    }
    cout << r << endl;
}