#include <algorithm>
#include <iostream>
#include <vector>

using namespace std;

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    int n, m;
    cin >> n >> m;

    vector<int> l(n);
    for (int& x : l)
        cin >> x;

    vector<int> h(m);
    for (int& x : h)
        cin >> x;

    sort(h.begin(), h.end());

    int w = 0;
    int done = 0;

    for (int d = 1; d <= n; d++) {
        for (int x = 0; x < l[d - 1]; x++) {
            while (w < m && h[w] < d)
                w++;

            if (w == m)
                break;

            done++;
            w++;
        }
    }

    cout << done << "\n";
}