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

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

int main() {
	int n , m;cin >> n >> m;
	vector<int> days , works;
	works.resize(n+1 , 0);
	days.push_back(0);
	for(int i = 0; i < n; i++){
		int y; cin >> y;
		days.push_back(y);
	}
	days.push_back(0);
	for(int i = 0; i < m; i++){
		int y; cin >> y;
		works[y]++;
	}
	int maxWorks = 0;
	for(int i = 1; i <= n; i++){
		maxWorks += (works[i] <= days[i])? works[i] : days[i];
		days[i+1] += max(0 , days[i]-works[i]);
	}
	cout << maxWorks << endl;
}