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

int main() {
	int n, m;
	cin >> n >> m;
	vector<int> days(n + 1);
	for (int i = 1; i < n + 1; i++) cin >> days[i];
	priority_queue<int> q;
	for (int i = 0; i < m; i++)
	{
		int a; cin >> a;
		q.push(0-a);
	}
	int done = 0, day=1;
	while (!q.empty()) {
		while (!q.empty() && 0-q.top() < day) q.pop();
		while (days[day] && !q.empty()) {
			done++;
			days[day]--;
			q.pop();
		}
		day++;
	}
	cout << done;
}