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

using namespace std;

int main()
{
    int N; cin >> N; //munkanapok
    int M; cin >> M; //megrendelesek

    vector<int> munkaorak(N);
    for(int i=0;i<N;i++) {
        cin >> munkaorak[i];
    }

    vector<int> hataridok(M);
    for(int i=0;i<M;i++) {
        cin >> hataridok[i];
    }

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

    int munkak = 0;
    int nap = 1;
    int orak = munkaorak[0];
    for(int m = 0; m < M; m++) {
        if(orak == 0)
        {
            nap++;
            orak = munkaorak[nap-1];
        }
        if(orak > 0 && hataridok[m] >= nap)
        {
            orak--;
            munkak++;
        }
    }

    cout<<munkak<<"";

    return 0;
}