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

int main() {

    cin.tie(0);
    ios::sync_with_stdio(0);

    int N, M;
    cin >> N >> M;
    vector<int> napok(N);

    for(int i = 0; i < N; i++)
        cin >> napok[i];

    priority_queue<int, vector<int>, greater<>> rendelesek;
    for(int i = 0, j; i < M; i++)
    {
        cin >> j;
        // cout << j << endl;
        rendelesek.push(j);
    }

    //cout << "asd\n";
    int megcsinaltak = 0;
    for(int i = 0; i < N; i++)
    {
        while(!rendelesek.empty() && rendelesek.top() < i+1)
            rendelesek.pop();

        while(napok[i]--)
        {
            if(rendelesek.empty())
            {
                i = N;
                break;
            }
            rendelesek.pop();
            megcsinaltak++;
        }
    }

    cout << megcsinaltak << "\n";

    return 0;
}