#include <iostream>
#include <vector>
using namespace std;

int main()
{
	int n, m;
	cin >> n >> m;
	vector<int> capacity(n);
	vector<int> dueToDayN(n);
	int res = 0;
	for (size_t i = 0; i < n; i++)
	{
		cin >> capacity[i];
	}
	for (size_t i = 0; i < m; i++)
	{
		int a; 
		cin >> a;
		dueToDayN[a-1]++;
	}
	int plus = 0;
	for (size_t i = 0; i < n; i++)
	{
		plus += capacity[i];
		plus -= dueToDayN[i];
		if (plus >= 0) {
			res += dueToDayN[i];
		}
		else {
			res += dueToDayN[i] + plus;
			plus = 0;
		}
	}
	cout << res;
}