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

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

int main() {
	int a, b, IN, work = 0, worked = 0, wanttowork;
	cin >> a >> b;
	vector<int> works(0),tobeworked(a);
	for(int i = 0; i != a; i++){
		cin >> IN;
		works.push_back(IN);
	}
	for(int i = 0; i != b; i++){
		cin >> IN;
		tobeworked[IN-1]++;
	}
	for(int i = 0; i != a; i++){
		work += works[i];
		wanttowork = tobeworked[i];
		if(wanttowork > work){
			worked += work;
			work = 0;
		}
		else{
			work += -wanttowork;
			worked += wanttowork;
		}
	}
	cout << worked;
}