// Source: https://usaco.guide/general/io

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

int main() {
	int days, works;
	int x, y;
	vector<int> daysVector;
	vector<int> worksVector;

	cin >> days >> works;
	for (int i = 0; i < days; i++) {
		cin >> x;
		daysVector.push_back(x);

	}

	for (int i = 0; i < works; i++) {
		cin >> y;
		worksVector.push_back(y);
	}
	sort(worksVector.begin(), worksVector.end());

	int cur = 0;
	int day = 0;
	int all = 0;

	for (int i = 0; i < worksVector.size(); i++) {
		if (cur == 0) {
			cur = daysVector[day];
			day++;
			if (day > daysVector.size()) {
				break;
			}
		}

		if (worksVector[i] >= day && cur>0) {
			all++;
			cur--;
		}
	}
	cout << all;
}